Macbook Pro 2016 15" model. The speakers often go from perfectly clear to very noticeably muffled when adjusting the volume. It often happens when turning the volume up - but it'll still be at reasonable levels. It gets muffled immediately as opposed to slowly degrading in quality which leads me to believe it's not a problem with the speaker slowly deteriorating.
I've experienced this issue and after a complete re-install and every troubleshooting regarding software tried out, I handed it in at an AASP. They determined the issue to be hardware, and exchanged the top case (which also means no more sticky keys).
So my best advise is, take a backup and go visit your local AASP.