CAT5 cable strands will work, plenum rated is preferred because it will typically have a higher melt temp. CAT5 tends to not take solder very well in some cases. Keep in mind that 24ga wire is only good to about 2A for short distances (10 feet or so), so keep that in mind if you want to use it for the run to the driver. (At 10', there would be a 1V drop with a 2A load, you could get away with more than that but the wires will get warm/hot)
see here
http://genuinedealz.com/voltage-drop.html
I have been using 18ga automotive wire rated 60v / 80C, it works quite well as it resists shrinking/melting when you tin it, but it gets pricey if you have to run all over to complete color strings.
I've been building 2 x 72 led fixtures for a friend lately and learned that serious forethought will save you big time... I ended up having 8 white / 4 blue per aluminum strip and to get the voltage I wanted to work at, I have 2 strings of 12 blue and 4 strings of 12 white, but to get those numbers I have to tie together 3 strips worth of blue etc. It sounds straightforward but it is a LOT more work than just sizing your power supply accordingly. I'm designing (and just about to order drivers for) a 198 emitter fixture and am being very careful to order the right drivers to save time.. In my case it means a HLG320-54 for whites (16 per row), HLG185-48 for blue(8 per row), LPF90-48(2 per row) for ocean coral and LPF60-15 for violet (3 per). Not only is it less work, it's more straightforward and I'll be less likely to screw things up. Plus it will be easier to troubleshoot if there is trouble. (total fixture wattage 598w)
I found some really nice 18g / 9 conductor wire for a good price on *bay, it is used for installing car stereo amps for 4 speakers. The outer jacket is black and thick, but the wire has good flexibility and 18ga is good to 7-8A with a low voltage drop.
Just to be clear, a voltage drop is
wasted power.
edit: Just realized I hadn't answered the question I was going to - yes, speaker wire will work fine, but that low temp vinyl (PVC) jacket on speaker wire will turn to goo when it gets hot.