One of the most common problems with the Sounddock is that the dock connector gets loose, giving different symptoms (depending on exactly which of the 30 pins are not having good contact). No sound, one channel sound, no control and similar are all common symptoms for this condition.
If you are VERY GOOD at soldering, you can fix this by resoldering the connector or replacing the connector. But most people can't perform this soldering in which case you need to replace the entire docking PCB. You can get 3rd party replacement PCBs at http://www.chargeconverter.com/shop/connector.htm?connbose.htm