Watching BB King sing he has the microphone set up on his left side. BB King is left handed but plays right handed on the guitar. Watching his lips he opens the mouth much wider on the left and seems to sing from his left side. It may be he is just responding to the placement of the microphone. I think he is demonstrating more vocal muscular control on the left. It reminded me of this video of the autistic boy who seemed to switch way to the right to open his mouth when he imitated Iago from Aladdin.
If I want to talk using my left tongue/side of head. I have to engage a great deal of musculature on the left that seems to have a much more passive role when I normally speak