Yes it is as long as you send the L-R signal over specific pairs rather than any arbitrary four wires - the twists in the pairs are tightly controlled and very effective at rejecting noise.
I have heard tale of audio being run successfully over 300m of cat 5 although for this distance you really need to use balancing transformers - so-called Baluns - eg:
http://www.canford.co.uk/Products/2006026/28-7028_MUXLAB-500028-Stereo-hi-fi-balun
They're not cheap (cheaper ones are available), but under some circumstances, using baluns and Cat 5 can be less expensive than running dedicated audio cable.