iMac internal microphone problems

I have a recurring issue with the internal microphone in my iMac. Occasionally, it will just stop working and won't pick up any sounds. The two resolutions I've found are to either unplug and re-plug the external speakers I have connected (strange) or, when that doesn't work, to restart the computer (less strange). Has anyone had a similar problem with their iMac? Adjusting the settings in the sound control panel does nothing to make this work. It seems that this is a software problem, since it's always fixed by restarting, but of course I'd like to find some way to fix this without having to restart. Thanks.