Lost Audio

I am using the free version (15.4.4445) on both my macbook air, and someone else's macbook pro. It works for a while, then I can no longer hear the other person. I can still see their desktop, we can still see each other, but the speaker icon under the other person's visual goes grey  with a large x next to it. It says to turn the speakers on. Neither of us have changed anything. The only way we can get the audio back is to shut down and connect again. Then a few minutes later, audio goes out again. Any thoughts?