Guess me and my tapeworm must go further down the road, 'Cause we eat so much, won't nobody give us no board - Me And My Tapeworm, Sylvester Weaver 1927
They didn't speak of the vocal division of labor, between the two players wax. Both of them sang, too. Sonny sang a lot, actually. I'd still contend that no disrespect towards Brownie was either expressed or intended.
Having watched it again I agree with MarkC who suggested it was likely a bug in the video editing suite that accounts for Brownie not getting his fair dues, a sin of omission in other words.