Hello Godpork,
Thank you for using Apple Support Communities! I understand from the information you provided that the right speaker sounds muffled; I'd like to help.
First reset the NVRAM on your Mac to see if that helps:
Reset NVRAM or PRAM on your Mac
To help further isolate potential causes for this behavior, I suggest seeing if this continues to be an issue when you restart your Mac in safe mode:
Use safe mode to isolate issues with your Mac - Apple Support
Testing in another user account can also help determine if this may be user or account related settings causing this issue. If you don't have another user account, here's how you can create one for testing:
How to test an issue in another user account on your Mac - Apple Support
Hope this helps.
Best Regards.