There are two problems with running speaker voltage down CAT5. One is that you have to run a hefty amount of current to a speaker, and it is possible under HUGE volume conditions to get the wire hot. This is kept in check by the other problem, resistance.
The resistance of the CAT5 is much greater than that of speaker wire. The net result is less volume out than put in, but also much less control of the woofer.
Speakers are motors, and once a woofer starts moving, part of what stops it is the very low impedance of the amplifier's output and the speaker wire. When the voltage is removed, this near-short acts to stop the motion of the woofer. The concept of the source's low impedance damping extraneous motion of the woofer is called damping factor.
With CAT5, the higher resistance will let the woofer move tiny a bit after the voltage has stopped, which also means the woofer will not move exactly in accordance with the signal from the amp. Audio will come out as a result of this extraneous motion. Audio that differs from the voltage put in is called distortion. Distortion is bad.
Will it be audible? Maybe. Maybe not. If it were a stereo pair for critical listening, it might sound muddy or the left-right and all other imaging would be smeared. Drum hits would not sound as crisp as they should. Is it for the surround speakers and your main unit is a home theater in a box? Wire away; nobody will hear the difference!
A good answer is easier with a clear question giving the make and model of everything. "The biggest problem in communication is the illusion that it has taken place." -- G. “Bernie” Shaw
