Audio problems Chrome/firefox/safari on an Imac

I’m not quite sure if I’m on the correct forum/page but I have a problem with the audio setup. For some resaon, my microphone doesn’t work: no one hears me. And also, I can see other guests but not hear them. But when I check the mute button (meaning the microphone pictogram is barred), I can hear guests but they still can’t hear me…
Any ideas are welcome. I have a meating in less than an hour.
Thanks a lot!