Heh, that’s a novel idea. I’ve thought about and heard suggestions on using OpenSprinkler/OSPi/OSBo for various other projects, such as Christmas light controller. But never thought about using it for sound system. I assume the sound system you are referring to runs on power line voltage. To reduce the complexity of wiring, I would actually suggest using RF remote power sockets instead of hard wiring. You can use OpenSprinkler coupled with an RF transmitter to wirelessly switch on/off power sockets. I’ve used this approach in several projects:
http://rayshobby.net/?p=3381
http://rayshobby.net/?p=7817
The nice thing is that a single RF transmitter can talk to several power sockets, so it’s pretty convenient.