Help - Mac Mini not sending audio via usb port

Hi,
I just bought a new mini and transfered music from a laptop with windows xp to the mac. When I run itunes on the Mac, the internal speaker of the Mac put out sound but my main audio speakers did not sound (dead silent). I have the Mac USB to PS Audio DL3, then from the PS Adio DL3 to the integrated amp.

When I replace the Mac with the Dell laptop with Win XP, the system works so it is not a cabling problem the DAC or the amp. Do I have do some settings on the Mac OS Leopard? Itunes on the Mac? Somehow the music stream does not come out from the Mac mini usb port.

I am also a Mac newbie .

Thanks