I had a similar problem with my A1200. If I pushed the sound connector or the power connector down I could often get the sound to come back. I thought maybe the socket was lose so I opened up the 1200 and checked. Everything looked good. I powered it up with the motherboard on the table and everything worked fine.
The only thing that I could think of was that the top section of the motherboard shield was shorting something out. I reassembled the system with everything except the top shield and I haven't had a problem since.
