There may be a couple of wires that could be used for something-- mabe an AM antenna. I would probably re-wire the house. Many times we don't get the luxury of wiring before drywall. Usually we can get wires through most walls without a single noticable incision in the drywall. This is one of the many things that separates us from hack electricians.

It also never ceases to amaze me at how many audio/video contractors don't know the first thing about speaker placement, wire, amplifiers, etc. Doing this kind of work should require a license with a test that requires audio/video knowledge. In California, you need almost no knowledge of audio video to be licensed to install speaker, data, telephone, or cable wiring. I guess electricians have a strong lobby.

Sorry I don't have an easy fix. Sounds like a can of worms to me. If you can't re-wire, I'd probably let some know-it-all electrican screw himself by taking the project on. I pity the cleint. No wonder the contractor died of a heart attack with subs like that.