Does the 5.1 surround system have an optical in? if so, you don't need to buy a sound card; you can buy a mini-optical to toslink adapter (Belkin and Apple do sell them, as do some other off-brand vendors, unsure pricing offhand but it'll be under 25 bucks I think.)
Otherwise, you need to be really careful choosing a USB soundcard to be compatible with macos. windows-only software soundcards won't be very useful. Pretty much any pro gear would work, but that's probably not cost effective or useful. The good thing is, most basic "usb sound" devices work without drivers, but I don't know any offhand which offer 5.1.


"Throw away logic and kick reason to the curb"