I have a PC in one room, surround processor in another connected via cat5 - I want to be able to play audio on the pc and listen in the other room.
Using something like turtle beach's Audiotron might be a solution, but I'd prefer to have the two connected directly.
If you want to do this properly you need to conver it to AES EBU proffesional. This standard is desinged to work with 110 Ohms (cat 5 is 100) but is very tolerant of the exact impedance because it was desinged to work over existing balanced micropone cables in studios and the impedance of these is very poorly defined.
However if you want a really cheap solution, just connect it up and see if it works, you wont do it any damage so long as you don't short the output out. You will have an impedance mismatch at both ends that will cause some ringing, this could cause it to misclock the input which would cause it to mute or make white noise, so try it with the volume low on your amp to start with. What have you got to loose it may work.
Otherwise you need an impedance transformer to match to the line and an amplifier to increase the output level then a recieving transformer and a attenuator at the other end to match it to your amplifier. you can probably buy these from the likes of Canford Audio (professional supplyiers) but you won't like the prices.
The other option if you have two pairs free in the cat 5 cable is to send analogue audio instead. This won't care about the exact impedance and CAT 5 sounds Ok as an interconect. Certainly as good as the output from a PC. The problem with this is you may run into hum, again that is a suck it and see thing.