FaceTime Audio quit working

My FaceTime callers can no longer hear me when I call them on my iMac running OS X 10.3. This wasn't a problem previously and I can't find any system preference setting errors. We see each other normally and I hear them but they don't hear me. My mic is not on mute and my internal mic is selected. Are there other tests I can use to determine if this is a software or hardware problem?

Thanks, Dale