cat 5 can be used for audio ONLY if the connections at both ends are "balanced" which , in car systems they are not.... (or at least i've never seen one that is)
balanced connections use a polarity inverted signal, a normal signal and then a differential input , canceling out any noise picked up enroute.. twisted pair with overall shield (as per cat V) is usually sufficiently comfortable with common mode rejection to give a good s/n result. so a single connection required 3 conductors... Hot, Cold, and earth, the earth being entirely separate from the signal, as opposed to unbalanced , where the shield is also the signal earth and is more susceptible to ground loop issues....
for unbalanced connections, as in most home hifi, and ICE systems, a decent quality coaxial audio cable, with double lapped copper shield is advised...
if any one needs lengths of up to 20 feet at a time, i have masses and masses of this lying around in the shed... in the form of no longer required guitar cables, and similar unbalanced line cables.
free to any worthy cause.
i may have the odd bit of fig 8 stereo cable as well, but most is single coaxial , which generally has better shielding and thicker insulation.
low capacitance cable is best, as it reduces HF loss over longer signal runs.