first off, robbie remixed nearly all the concert, as he wasnt happy with the recording. second, youre probably hearing the other backing singers, such as danko or manuel, which the camera is raarely on. And yes Levon was bitter, because the only one who was so commited to the finality of the Last Waltz was Robbie, mainly becasue his creativity was running out. The others wanted to keep going and later reformed without him.
I like the story of how Ronnie Hawkins, after attending the premiere, stood up in the theater and shouted, "That was good, but it could have used some more shots of Robbie!"