There is a bit of a difference between throwing together a proposal for a theater room and 4 zones of audio vs full on system design.

Either way I would hope this stuff is factored into your cost of business and figured into your pricing. If not, I don't know of too many other trades that work for free. Add on the fact that a client that expects you to work for free probably isn't going to be a client that gets you referrals down the road either.