Firstly, try some basic troubleshooting - the mic ports can be sensitive to how well the mic is plugged in. Unplug it and plug it back in firmly and try again. If this doesn't help - try the other microphone port. Do you have another SX20 that you can temporarily swap mics with to see if the problem follows the mic or the SX20? If non of that helps, it's likely you have a faulty Mic or SX20, or you're hitting one of the problems I encountered with bug CSCuw74729 where the mic mute button just doesn't work following a software upgrade/downgrade and the endpoint needs to be RMA'd. What's the hardware and software versions of your SX20?
Wayne -- Please remember to rate responses and to mark your question as answered if appropriate.
Wayne -- Please remember to mark helpful responses and to set your question as answered if appropriate.