You can use CAT5 cable as a whole for each wire to the speakers (two for each speaker). But this is expensive proposition unless you have access to the source of free cable. If you still make this choice, just connect all individual wires in CAT5 together on both sides. That way each CAT5 cable will service as a single wire.
I think what you are going to want is a constant voltage system.
Yes a pair of 70 volt transformers for each channel could work, but if you get high quality enough transformers, they would cost as much if not more than the more obvious solution which involves a power amp at the speaker end of the CAT 5.
I was thinking single channel PA amp with appropriate speakers. The transformers on them don't need to be particularly high quality for voice. There are lots of affordable 70V horns that will do the job.