I think the pipe under the slab is a great idea!!! Don't let it be anything less than 3/4 inch diameter. This will keep your cable runs as short as possible.
I would use a 12 gauge generic speaker wire and spend the money on something else. The insulation should have something like a CM rating. It doesn't have to be rated for direct burial.

Something else while you are building is to run a piece of 2" PVC pipe from the first floor to the attic if possible. This is for future electrical circuits. Good Luck Jim