NOTE: I automatically presumed you meant STEREO sound. If you meant surround, then obviously, the solution below is not applicable. However, I will leave the post intact as it is still a valid recommendation for stereo application(s).
Do you want to play around or get serious?
I am going to recommend a very useful, serious tool, that is a premium level performance product: E-MU 0404 USB(NOT THE PCI CARD; which is a totally different and inferior product). This device is not only a top quality DAC, it has top grade ADC, and just about every analog and digital I/O one could want, and unbalanced and balanced analog, and it also works stand alone (!) without a PC as an I/O converter and mic pre.. It also has top notch mic pres with 48V phantom voltage. It also has a dedicated high quality headphone amp built in.
Basically, it does almost anything you could want, and it does it all superbly. BTW, you will find the phantom voltage powered mic-pres useful in the future when you want to get serious about setting up your audio system.
-Chris