unable to be heard by others when on Zoom

Hi there, when I connect onto Zoom on my Apple IMac, I can hear others, but they can't hear me.

When I did the input level check its very faint.

I've checked security / privacy settings and have ticked the Zoom box.

I have recently had the logic board replaced due to a different issue and contacted Apple Support who said it wasn't the microphone as when I tried using FaceTime, I can be heard fine.

Not sure what to try next???