Running baseband analog audio signal over cat 5 cable
Your question is impossible to answer quantitatively since the level and type of interference along the cable is unknown, and you haven't told us how the end is terminated.

Look up the resistance per unit length of the cable you want to use.  From that you can calculate the resistance in series with your signal.  Don't forget that the signal makes a round trip, so the total wire length is twice the cable length.

That all said, sending single ended audio over a long unshielded cable sounds like a really bad idea to me.

My first reaction would be to send the data digitally.  That's what cat 5 cable is intended for.  You can easily send a few Mbit/s over one pair.  To put this in persepective, 16 bit samples at 50 kHz rate (better than CD quality) is only 800 kbit/s.

Digital gives you a certain noise immunity directly.  It also allows for easy common mode rejection at the receiving end, by using opto-isolators or pulse transformers.

My second choice would be to use one pair of the cat 5 cable to send differential audio.  I would make the voltage higher than you suggest.  That makes it larger relative to the inevitable noise along the way.  A small audio transformer at the receiver is a easy way to eliminate most common mode noise, and that also removes the need to for a third wire to carry ground.