Input volume keeps decreasing

Has anyone experienced in the input volume turning down automatically on their imac's and mac air products? Every time I zoom or skype someone the computer reacts to their voice and automatically reduces the sound to zero, so they can't hear me. I find I have to hold the button down throughout the call, so that they can hear me, which is really annoying. Is there anyway to turn off the automatic setting and override this so that you can keep your sound level constant?