Webinar Audio Quality Poor

We hosted a dress rehearsal for some webinars we are hosting through Zoom. We are hosting the webinar in a multi-cam studio, our subjects have lapel mics, and all the video and audio feeds are being fed through a Blackmagic Atem Switcher. 

The video and audio is being sent through to a Mac Studio, which receives it as a camera input. This is coming through to Zoom.

Monitoring the audio that is being received in the studio control room, everything sounds great. The audio being received on the attendee devices however, is very muffled and barely distinguishable. We have tried through different devices - mobiles using the app and laptops as well. 

I just wanted to check to see if there's some basic setting that might need changing or something? I have the background noise suppression set to low, and when I change it to anything else it creates a high pitch ringing/feedback noise.

There is a small chance something is affecting the audio before it goes through to Zoom, but I find that unlikely.

All help is appreciated - There's just under two weeks until the first webinar session.