CAT6/5 cable is not adequate for driving speakers. I suggest that you just use the affordable 12 AWG wire. For the lengths required, gauge 14 should be amply sufficient.
For in-wall wiring, you have to use the CL2 rated wire in accordance with building codes.
Here is a good affordable one. I believe Amazon also sells it:
Get the most out of your home audio system with high quality, pure bare copper speaker wire from Monoprice™!Pure Bare Copper is a superior conductor to the copper clad aluminum (CCA) conductors
www.monoprice.com