How well would cat-5/6 work as a multi pair audio cable?
Should be fine over short distances.  If the cable is going to be in a noisy environment (lots of 50/60hz EM especially) go with shielded CAT5 and make sure the shield is grounded properly.

If your looking at longer distances, consider differential signaling, which again the twisted pairs of a cat5 cable are just fine for.

As always when dealing with patching devices together like this be careful of creating ground loops.