Just happened to me, worked fine last night, this morning balance was set all the way to the right:
Model Identifier: MacBookPro13,3
System Version: macOS 10.13.4 (17E202)
Model Identifier: MacBookPro13,3
System Version: macOS 10.13.4 (17E202)
These are not separate settings, they control the same thing as the general balance slider in the main audio settings. You can have the MIDI window and the audio window open at the same time and watch one influence the other.Go to Applications > Utilities > Audio MIDI Setup
Select your output device on the left and make sure both Channel 1 and Channel 2 have the same dB values.
Still not fixed as of 10.15.5.
My mac exhibits this problem with both internal and external speakers. From what I read, this is caused by rapidly pressing volume up/down button when the cpu is under heavy load, that's why I'm inclined to believe the issue is independent of speaker types...Do you have USB speakers? What I've come to learn overtime is that many external USB speakers end up having this problem.