The speaker buzzing sound has been haunting me as well, but mine is always the front passnger door speaker and it is getting extremely annoying, it quite bothers me... here's my experience. I have a 11/2016 MS with premium sound.
My buzzing speaker is the front passenger side door speaker. It started about a year ago and it was intermittent.
January this year I took the car in to the SC for the FSD computer upgrade and added it to the list.
They replaced the RH Condenser Fan. (see screnshot - I don't know why the test drive... driving has nothing to do with it) and stated that noise was gone... well unfortunately it was intermittent and they thought they fixed it, but it came back.
It kept getting worse as far as frequency, and at one point it bacame a permanent feature. It buzzed anytime the screen was on.
Then in August took the car in to get the MCU2 and again I pointed out the buzzing sound. This time they replaced the speaker (see other screenshot) and clamed it went away. BTW it is best heard without music playing becaus the music covers it up...
Anyway, the MCU was also replaced and the buzzing had gone away. Yay!
Well that was August, and the excitement of driving a buzzing free car quickly faded, as the buzzing sound started to come back intermittently at first, and now, only about 2 months after MCU upgrade the same buzzing sound, fram the same speaker is back, and it is no longer intermittent, it is once again always present with the screen on. Nothing I can do to make it go away.
So hopefully soon I get the invite to get the new camera upgrade, and once again will add the buzzing sound to the list of concerns, but seems like they have no clue as what causes it.
Without music playing it is very annoying, with passengers annoyingly asking (what is that sound!)
I hope something is found...