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.
http://www.studiohub.com/Here is a page that has pinouts of their system:
http://www.studiohub.com/PDF/PinoutGuidewithAdapters.pdf
