Cat 5 is designed for things like computer data transfer where each pair has separate data signals traveling through it simultaneously. The only significant difference is that each of the pairs are twisted together to help 'insulate' each pair from the other pairs signal via induction.
It is the twists that make this a better choice for data transfers, it is not the 'quality' of the cable. Without the twists effecting the magnetic fields to reduce induction, you would be limited to very short lengths for data transfers. This is not nearly as relevant to a single pair of speaker wires, even if they are mixed in the wall with a few other speaker wires.
Not being argumentative at all, just thought I could help clear up this point.
Great point Murph. Cat 5 makes a terrific speaker cable because of its low inductance and low resistance.
One of my pet projects a while ago was to find a set of speaker cables which didn't colour sound at all. In a closet somewhere I have a boxful of various exotic cables some pricey, some not. Regardless I always came back to single 12awg zip chord, until I came across the article below. So I made a DIY set of in-wall 16awg doubled up cables (becomes 13awg) with ultralink connectors. I've been very pleased with the results.
Cost for 1 set of 16 ft cables was $20 for the in-wall dual connector twisted quad electrical cable at Home Depot, $20 for the connectors, $1 for the shrink wrap. $41 in total. As the article said a main objective is to keep inductance as low as possible the longer the run, hence the advantage of dual connectors.
http://www.hometheaterhifi.com/volume_10_2/ultralink-CL414-cable-5-2003.htmlAs for interconnects that's a different story, but as the Colin Miller commentary suggests the better ones have low capacitance characteristics. The Blue Jeans Low Capacitance interconnects are what I use for RCA connections.
http://www.bluejeanscable.com/store/audio/index.htm 
