I have the same issue. Audio settings no longer change the audio. All of my zoom lessons have been bounced from zoom as of now, due to the zoom audio issue with piano. 

It is not a systems issue, as I have students facetime me and its perfect, no issue at all.

However, I travel abroad in two weeks and during that time I will only be doing zoom lessons, and if this is not resolved, I don't know what I'll do to maintain my work week.