Thanks for the tips. It is unfortunately a set design from a large development group with minimal ability to change any thing remotely structural. We did run Cat6 to all foreseeable needed areas and dedicated 20A circuits and outlets, but no go with getting to do anything like in walls.
Condolences to you on that. Those sort of builders are generally the worst of the lot. Make sure the building inspectors go over it with a fine tooth comb.
Seriously though I would check on things like the roof truss specs and everything you can think of. I mention that as I had to totally re-speck the trusses. I have just done a solar project and even with the added weight I still have 100% unused load factor. The solar people showed me photographs of other homes and some could not take the load of the panels. What is saw was a bunch of roofs ready to blow off in the first really high wind.
Remember you are the best inspector. If it looks dodgy it will be, so make them do it properly.
If you can go in wall, then at least pre-wire speaker and equipment locations with conduit.
If you want in walls, I'm sure they would leave a wall, or part of a wall unfinished. In addition there are commercial in walls that can be placed in finished walls, and you can pre-wire for such.
The problem is great rooms are large spaces that take significant power, and speakers to do the job can certainly detract from the space. The other option is to create a dedicated AV space. I have the luxury of being able to do that as well.