Nope - Cat5 is 4 twisted pair, which makes it somewhat noise immune due to the twisting, but no shield that I'm aware of.
the only reason you can run 100 Mbit on it, is that it's most likely pretty low capacitance. Most digital cable is, so it doesn't round off leading/trailing edges of pulses as much.
You might get away with using it for XLR's until you get a cable too close to a noise source, but I wouldn't recommend it.
I have heard of people using a Cat5 patch bay and cables for digital audio, since the real thing is so expensive - I'm not sure how/where they converted to the RJ- series connectors though. I personally wouldn't recommend even that, after reading about jitter, etc., on sites such as these -
http://jitter.de/english/engc_navfr.html
http://www.digido.com
Maybe the best thing you could do with all that Cat5 cable is get a few pounds of RJ conn's and a crimper and sell short Cat5 cables, use the money to buy XLR's. That's what I would do, if I didn't need the Cat5 for its intended use... Steve