People can't hear me on my Samsung Galaxy S3 When I send or receive a call, the person on the other side can't hear me. 

Sometimes, the microphone goes mute during the call. But when I check if the phone is on mute, but it isn't. It happens randomly.

The microphone seems to be OK, and I think this is probably a software problem. Would upgrading the OS (from ICS to JB) fix this?
I  was a having the same problem, look next to the charging port (that would be to your right) there a tiny hole CLEAN IT OUT it is blocked. I used a sewing needle to clean it out. After that people on the other side could hear me again. That hole used for talking.