How to disperse avail. amperage?

Lissnr,
You did not mention how far from the house the garage is.
Distance from the house main electrical panel, to the new audio room sub panel? You may need to take into account voltage drop.

You stated the new sub panel feeder will be buried 18" below grade.
Direct burial cable or PVC conduit with single conductors?

I would recommend PVC conduit over direct burial.

Copper or aluminum feeder conductors?
Copper will cost you an arm and a leg.....

And I hope you are not thinking of an isolated dedicated equipment ground rod.

The equipment grounding conductor shall occupy the same raceway or cable as the feeder conductors and shall terminate in the same panel as the feeder conductors, NEC.

I'm expecting the main breaker on the house side of it to probably only be 100 amps? as well as the main breaker on the sub panel to only be 100 amps too... mostly because

First off you do not need a main in the sub panel, that is unless your local code requires it. If your local code does not require a main then all you need is a main lug only panel.

The sub panel and sub panel feeder will be protected by the breaker at the main electrical panel for the new feeder.

As for the sub panel what brand are you looking at?
I would use a panel with copper bus. If you have the money I would go with a Square D NQOB panel board. The bus is copper and the breakers are bolt on.

I'm expecting the main breaker on the house side of it to probably only be 100 amps? as well as the main breaker on the sub panel to only be 100 amps too... mostly because the 100 amp breaker is 1/3 the $300 cost of the 125 and I can't imagine I'd be drawing that much current all at one time?

I do not believe you could install a 125 amp breaker in the main 200 amp electrical panel anyway. It might fit but the panel is not approved for that large of a breaker.
Curious, did the electrician say he could?

Thank you both for your replies, between my electrician's visit today (I picked his brain for well over an hour) and the info. I'm getting from both of you, I'm learning a lot more than I've ever been aware. The biggest lesson I now understand is that a subpanel rated at a certain amperage (ie 125A) can have many more breakers on it, simply to dedicate the individual items, than the total of all the breaker amp ratings combined. This opens up considerably more flexibility regarding lights, A/C, dedicated audio lines, etc.
Gs5556: I will do the breaker designations as you described, namely 5x20A dedicated audio, [probably] a 20A for the A/c unit, and a 15A for the lighting. I also mentioned to my electrician that I was considering the possibility of adding a 1/2 sized refrigerator (like the ones in hotel rooms/college dorms) and he said I should put another 20A breaker dedicated for it alone... (I thought I'd mix it on the A/C line and he said the A/C should have its own circuit...) If I took this whole concept to the max, so to speak, does that mean I could conceivably add one more 20A breaker for "future potential" home theater items such as a surround sound receiver/amps, projector or big screen LCD TV, etc??? This is the time to ask right? No, I can't think of anything else...That would make a total of :
1x15A lights
1x20A A.C
5x20A dedicated audios
1x20A refrigerator
1x20A home theater
and thanks again for the kind words : I hope it does add value to the house!
Jea48: the house panel to garage distance is only about 40' in a straight line BUT I don't have the option of digging that trench for the cable straight due to trees and a patio in the way. My electrician recommended that I run the main (indoor type) wire from the house panel along inside my patio soffit for a distance of 65' until I get to the corner of the house where I can THEN run a junction box of sorts to another 45' of the heavy duty outdoor direct burial cable in an unobstructed trench to the sub-panel placement point. I am not looking forward to delving into a perfectly neat soffit to do this but I think I'm out of options. All cable will be copper.For a few $$$ more you'd recommend housing the direct burial cable in PVC? OK, can you estimate what I.D. PVC I will need? I don't know what size breaker he said he'd mount at the house panel... it won't take a 100A? He kept trying to talk me into exactly what Gs5556 advised: a 2 pole 60 amp breaker... maybe I should listen to the experts instead of carrying on like I think I know what I need....(but if I add the home theater and small refrig options as above is the 60A breaker still OK?)
As for grounding, I asked him if I could run a grounding bar (or two) from the grounding point on the sub panel (yes, I am looking at the Square D QO series... I think it's the
QO11224l125G which I initially picked out on-line). He said I could run 1 or 2 if I wanted, spaced 8' apart both on the same bare copper cable (did he say 8G or 6G???-I'll ask again...). I did not mention to him that I wanted to run Hubbel 5362 I/G's and run each ground wire individually back (to the sub-panel I assume??) using individual 10G solid copper wire for each and every terminal [and that's what's going to the grounding rods] [from the sub-panel]. I know it's labor intensive but from what I read on these forums it's supposed to be worth it and I was going to do all that wiring myself, then the entire job would be inspected by the town inspector (who my electrician notifies for me once he's checked all my work first). This is what I'm hoping... maybe he will let me run the Romex through the studs but won't let me make the actual connections? I'll find out later... right now it's "part 1: get the power to a sub-panel". Input/comments/help/corrections = highly encouraged! and thanks already for all your help fellas. Best regards...

Hello again, Jea48: OK, I'll tell my electrician that a 2 pole 70A breaker at the main box is good. When you say that the 2 hots and neutral on the main feed from the house should all be 2AWG copper and the ground should be #6AWG, for a total of 4 individual cables from the house panel to sub-panel? I didn't even realize that the ground wire was supposed to return to connect to the main house panel!!
"The sub panel will have a separate equipment ground bar bonded, connected, to the metal panel enclosure."
What do I know? I thought we were going to run a ground wire from the sub-panel via a #6G bare copper wire to an 8'-10' pole hammered into the ground at least 8' from the garage and that was THE ground... Do I still do that, even with the #6G returning to the main house panel [underground and in the soffit] along with the 2 hots and 1 neutral #2G's? BTW, if I use a PVC tube to encapsulate the underground wires will that be a different type of wire/can it be the same as the "in-house/soffit type" as compared to an "underground feeder cable"? I was checking prices on #2AWG copper cable and we're up to at least $2.50 per foot! Does that mean 65' + 45' = 110'x3 runs (2hot,1neutral) for a total of 330' x $2.50ea = $825 plus the 110' of #6 ground @ $xx? per foot? WOW...
Also, when you say the dedicated branch circuit will have 1hot,1neutral,1gnd... you are referring to the individual wire of 12/2 or preferably 10/2 Romex which attaches to the outlet? When you say "No sharing of neutral" what does this imply? I was thinking of running 2 x 5362 Hubbels next to each other 9for a total of 4 plug in points0 per 20A dedicated line... and I was going to place 3 of these on the wall behind my speakers to hook up all the amps, subs, etc, with therefore 2 add'l outlets per 20A line... Does this work in theory? Thanks for all your help.

I didn't even realize that the ground wire was supposed to return to connect to the main house panel!!

Yes it does, as per NEC.

I thought we were going to run a ground wire from the sub-panel via a #6G bare copper wire to an 8'-10' pole hammered into the ground at least 8' from the garage and that was THE ground..

Have your electrician read NEC 2005 250.54 Supplementary Grounding Electrode. Page 70-103

Do I still do that, even with the #6G returning to the main house panel [underground and in the soffit] along with the 2 hots and 1 neutral #2G's? BTW, if I use a PVC tube to encapsulate the underground wires will that be a different type of wire/can it be the same as the "in-house/soffit type" as compared to an "underground feeder cable"? I was checking prices on #2AWG copper cable and we're up to at least $2.50 per foot! Does that mean 65' + 45' = 110'x3 runs (2hot,1neutral) for a total of 330' x $2.50ea = $825 plus the 110' of #6 ground @ $xx? per foot? WOW...

I said it would cost you an arm and a leg.....
You could use aluminum. You would need to increase the size to #1 awg, and the equipment ground wire to a #4 awg.

The wire from your main electrical panel in your home to a junction box mounted on the house above the earth could be insulated THHN/THWN copper installed in 1 1/4" EMT metal conduit. You could use 1" EMT per code but trust me it is a bitch pulling 3 #2 and a #6 in 1".... From the JCT box through the earth to the new sub panel XHHW insulated copper wire in
1 1/4" PVC conduit.

Also, when you say the dedicated branch circuit will have 1hot,1neutral,1gnd... you are referring to the individual wire of 12/2 or preferably 10/2 Romex which attaches to the outlet? When you say "No sharing of neutral" what does this imply?

Multi wire branch circuits.

You can have two 120V separate branch circuits sharing the same neutral.
Each Hot conductor must be connected to opposite Lines, legs, in the sub panel. One on L1 the other one on L2. With this wiring configuration only one neutral conductor and one equipment grounding conductor are required. Two separate circuits..... But not two dedicated circuits.

I was thinking of running 2 x 5362 Hubbels next to each other 9for a total of 4 plug in points0 per 20A dedicated line...

Sounds fine. Many will recommend using plastic rough-in boxes instead of metallic.

Jea48 You have been a guiding beacon in all this. Due to the absurd costs of running 110' of wire through the soffit/underground route I have decided to bite the bullet and attempt the direct route under/beside the trees and under the patio... it's going to be a lot of digging but the soffit looks far too complex to pull apart, not to mention the direct route will save $$$hundreds. I haven't measured the exact distance but it should be between 40' and 50'. As per your recom. I'll probably make it all XHHW #2 insulated copper wire all the way (No need to bother with a junction box just to save a few bucks on the 10'-15' or so that's inside) and then place the external/underground section into 1 1/4" PVC to the sub-panel location. The rest of your advice is very very helpful too (running multi outlets off of each dedicated line), I'll keep you updated. Thanks very much! Best regards and happy Lissn'n.