I have the same issue. Only along with a pretty drastic volume drop, it also loses quality and sounds muffled.
It is also random, which I can't figure out. Including my own personal music that I created on THIS Mac (M1 MacBook Pro) and uploaded to my distributer.
I have spent HOURS with customer service and was even ghosted on by a specialist that was supposed to call me on a scheduled call, which I'm still not too happy about.
There are things that I have discovered trying to figure it out...
It has to do with the user account somehow. If I create a NEW user account and log into all my Apple ID, it plays ALL the music fine.
But this was NOT a viable solution.
I tried it, but since I have had the same user account that I have migrated from Mac to Mac over the years, it was too frustrating when every time I tried to do something I was having to redo settings and preferences, etc.
Back in my normal user account, the affected songs will drop quality whether streaming or downloaded playing in the Music app.
However... If the affected song is downloaded and is NOT an Apple Music song, but something I actually physically owned and uploaded/synced to Apple Music, then I can go into the Music folder and play that downloaded song in a different player, like Preview for instance, and it plays fine.
So it has something to do with the settings of the Music app on my particular User account.
It also doesn't seam to matter whether bluetooth to my AirPods Pro or Max, or my AirPods Max or external computer speakers wired direct to the headphone out, or using the internal MBP speakers.
If Music could be reinstalled, I'm sure that could be a solution. But it can NOT and I have gone as far as reinstalling MacOS (currently Big Sur 11.5.2) and the Music app on that particular User account is still having issues.
I have given up on the problem but I was just curious as it's been a moth or two since I last looked into it and I just found this thread.
Hopefully this info is helpful to someone and this can get figured out.
I'm also hopeful that Monterey will fix this issue. But it has not been fixed with updates to Big Sur so who knows?