I have macOS Big Sur and this issue has been plaguing me since installing it on my MBP 16". The temporary solution, posted some time back in this thread, is to open Activity Monitor and search for the process coreaudio (it's called coreaudiod) and kill the process (Force Quit). It will restart automatically. This avoids having to reboot your Mac every time this occurs.
Infuriating is an understatement.
2point5
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So I am not the only one getting this really peculiar error. I can't for the life of me figure out why and given that the code has worked perfectly before I updated to Xcode 12 I am at a loss as to where the issue is or if in fact it has anything to do with my code at all.
If you have a look at Activity Monitor does that show SourceKitService hogging the CPU and are your fans running flat out?