I have the opposite problem: I can hear my wife from an iPhone4, but she cannot hear me. The microphone icon is ghosted (with a line through it) and it unresponsive. Kind of embarassing that the iOS app works better than the Android version.
I checked the site recommended above with no luck.
I have the same problem, in other forums people are saying the same thing. Its a bug and Google is aware, and said " a fix should be out soon", but it been at least two weeks, so define soon. Oh and the Google+ update that was released earlier this week did not fix the problem.