Sounds like a plan coming together!

The 12 gauge wire should be fine. You are going to blow your ears right off your head before you can actually use enough current to ever melt them, assuming your runs are not way too long. There are lots of good articles regarding speaker wire length vs. gauge out there.

For in wall, your building code may also require that you use sheathed, in-wall rated cable.

With great power comes Awesome irresponsibility.