This is my first post, but I have been reading this forum for quite a while. I think I am experiencing the same intermittent "phasing" issue described.
I teach Ham Radio Classes using Zoom and YouTube (Channel name: W4EEY). Our main switcher is an ATEM Mini Extreme ISO. This switcher feeds Zoom via the USB C Webcam interface. We use two Sennheiser XSW 2-ME2-A Wireless Microphones each feeding Mic 1 and Mic 2. The mikes are Mono and the same signals are applied to the Left and Right channels. We have been monitoring levels using the metering and headphones.
A student recently complained that the quality of Mic 1 was much worse than Mic 2 on Zoom. Investigation showed that the problem is being heard in Zoom, recorded and also passed to my YouTube Live Stream via a second ATEM Mini ISO.
Playing back the recordings and viewing the audio channels on an XY display shows that when there is a problem Mic 1 is not a diagonal line (as it should be) but has widened circular characteristics (as if it were stereo). I have screenshots but I can't figure out how to post or attach them here.
A video with the audio problem is this one:
https://youtube.com/live/RpDWQYQE7joA normal video is this one:
https://youtube.com/live/3ESrh6fi-YoBoth use identical set ups. The normal video was shot one day later than the first.
I have now installed an extra sound card and can monitor channel phasing in real time from the ATEM Extreme's Headphone output and a Sound Card Oscilloscope.
We do have multiple cameras but we do not use the audio feeds from the cameras. But if I purposely turn on audio from a camera's built in microphone I do see a similar circular XY waveform (natural as this is a stereo microphone on the camera).
This problem is intermittent. Our next class is Tuesday night. I will be watching for the problem and will report back if I find something.
73 de W4EEY