fitness/meditation classes sharing audio

[Zee1973]

My goodness, I hope somebody can help me here. I'm trying things that I've spent 3 hours reading about and already tried. 

I have been using zoom for fitness/yoga/nidra classes for 3 years no problem. 

I got a new IMAC this week to replace old one and all of a sudden zoom isn't allowing music at the same time as my voice teaching. My voice quality is terrible if music is playing, I've tried to sharing musician thing being on etc. Please, I really welcome your input here, even if you think its seems obvious. I've got a ticket drawn up with Carla and sent a sample of how crap it all sounds, and has heard nothing now!