It's tough to answer specifically without more detailed information about your configuration. I'm going to assume your 7.1 receiver is a multi-functional device (tuner, DAC, preamp, amp) and accepts a digital coax input.
In that case, you would need an internal or external soundcard that provides a digital coax output. FWIW, I use an Audiophile 2496. Simply connect the soundcard's digital coax output to the receiver's digital coax input and you're all set. This requires a 75-ohm cable with RCA connectors on both ends. You'll then control your playback from the computer, using your software of choice. I like Foobar2000.