I am having the same issue - I am a musician, I can tell you that the sound quality has significantly decreased compared to even the 2019 13 inch MacBook Pro. The sound on the 13 inch M1 MacBook Pro sounds a lot more muffled and certainly quieter.
I would have thought that this would have been a software issue as opposed to a hardware problem with the speakers - It could be a simple software bug or poor audio tuning from Apple.
I must say though, I didn't do a new setup of the M1 MacBook Pro when I got it, I did a time machine restore thing from my previous 2019 13inch MacBook Pro which was running Catalina - I was thinking that maybe time machine has copied audio (/EQ/MIDI) settings from Catalina over to the M1 MacBook Pro that are not compatible with the M1 :(