Greg, That would depend on the distance that you were looking to send that audio, would it not? I see there are a heap of "adapters" now that allow you to send pretty much anything over Cat5 wire. These are powered units so you'd think there would be some sort of amplification/attenuation in there.
Line level audio on a cat-5 cable system is very simple-- pick a pair, hook an audio source to it, hook the same pair to the destination input. Broadcast radio has been doing it for several years. Here is a link for one manufacturer of audio to cat-5 components.
We used this wiring in a radio broadcast studio. There were no isolation transformers, just ran the wiring from point to point between devices (receivers, distribution amps, mixing consoles, etc.) We used shielded cat 5 wiring, the audio sources were balanced line level.
Telephone networks have been running line level audio on twisted pair for many decades.
That's the key to what makes it work. Audio source and destination equipment MUST be balanced. Stuff that's unbalanced must be made so with transformers or balanced-to-unbalanced amplifiers.
I helped wire a four-station cluster in 2006. Audio interconnects between studios and the rack room were made with overall-shielded 25-pair CAT5 cables. AES/EBU digital audio was sent over the same cables, since the impedance of AES audio gear (110-ohm for the balanced version) is essentially the same as CAT5 cable.
Back in the old days we did it with lo quality lines over huge distances, we called it telephony. One of the main issues is to get the right impedance, close to 600 ohms seems to turn out well. (impedance, not resistance!)
I was told by one of my engineer buddies that audio might as well be DC at the distances we are usually dealing with. Even 20khz (that I can't hear) will have a wavelength of almost a mile so I am not sure you will see any problems.
