Is there a way to improve the sound?

The NDS works much better with files ripped and stored as WAV files.  I heard a comparison of WAV and FLAC ripped from the same CD and the WAV files actually sounded a little bit better.  NAIM's dedicated ripper, the Uniti, rips to WAV.

You can get good sound from digital if you use uncompressed file formats. The problem with WAV files is there is no Metadata (track name etc.). FLAC is uncompressed and maintains the Metadata so I would recommend FLAC. Use Dbpoweramp or Exact Audio Copy for a PC and rip CD's to the hard drive. If you are taking CD's and making MP3 files you will easily hear a difference in quality because MP3's are compressed.

My bet is that your system is ripping MP3's to your hard drive. Ouch! Those are compressed files and they remove lots of sound from the file. It's like the difference between cartoons on cards that you flip so that the characters appear to move (and your brain fills in the gaps) and real life. 

Having said that: Nothing beats perfect vinyl from a great phono, it's the ultimate in an uncompressed file (it's beyond a high definition CD file), but there is a place for Digital. I remember the first Audiophile I ran into as a kid. He showed us his system and had a reel to reel tape player. He recorded his albums and listened to the reel to reel casually, but when it was time for a real listen the albums came out. He explained that by doing that he preserved the integrity of the albums. Additionally, iIt has some other advantages: no pops, clicks or scratches etc. The ability to skip from completely different songs and/or albums easily.

I'm an old fart too and have had to educate myself on this stuff - I still need more work.

BTW on another thread you asked about the Oppo. It's a good player. An incredible value for the money and nearly bullet-proof playing anything you throw at it. Some high end players can be a bit picky. 

Hope this helps. Email me in Audiogon if you need help - I'm not a dealer and I don't have anything to sell. Just trying to help another "old guy."

There is nothing about ripping and streaming that is simple. Ripping to a lossless format is a small part of the quality equation but there is a host of other factors to consider. Just to prove it to yourself, rip one of your reference cds on a laptop you have lying around. Then download the free version of Fidelizer and run it in purist mode on the same laptop (next time you boot changes made by Fidelizer will be reversed). Re-rip the same cd (unplug the laptop charger for good measure). Compare the two rips. This little exercise should prove to you that ripping and playing from a server is just as tweaky as setting up and playing vinyl and we are just on the cusp of figuring it out.

I listen to turntables as well as digital ,for someone to make a blank statement records are better is total system dependent 
dynamic range far better at least 20-30 db more capable 
the most bits a turntable can produce is roughly 12.5 not even basic cd standard ,noise floor S/N ratio not even close the analog 
sound and resolution good digital has very much so .i play my flac
files directed through my iPad off a dedicated player 
for example a Outstanding buy Schiit audio ,sold direct.
the Multibit Gungnir, $1300 dac, or their top dog 2,300
these multi bit dads are their design 21 bit is max that can be extracted at the moment . Go read their white papers .these 24 bit 
dads,and 32 bit are all in theory. It is what is the end readable amount Mike misfit ex Theta digital engineer using their own design using Analog devices chipsets. Are without question 
unbeatable even at 2-3 x their prices and many awards.
that being said you need quality cables , power cords 
and digital cords .now these little regen units clean the 5v that corrupt the digital signal. Up tone regen plug it in and here the difference.with these days or , the Vacuum tube$5k Lascala ,
or $7k . Lampuzator any of these will player beautiful analog sounding music ,without the hassle !!