Even I have this issue.

Here is the sample code with which the bug can be reproduced. It works flawlessly on XCode 12 but not on XCode 13 beta https://github.com/JoshuaSullivan/MetalKernel

I already filed a bug FB9393169 and the status is updated as "Resolution:Potential fix identified - In iOS 15". Not sure what this means. Because if this will be fixed in main iOS 15 release, then I don't need to do anything. But if it will not be fixed in iOS 15.0, then I will need to immediately update app and disable stereo capture else I risk lot of 1 star reviews.

Broken in iOS 15 beta 6 as well!

Still not fixed in iOS 15 beta 6!

Broken in iOS 15 beta 5 as well!

Still not fixed in iOS 15 beta 5. Looks like iOS 15 main release is going to have broken stereo audio API