I agree with Kathryn, it could be a regional thing. I have never really noticed mmmbye. I have noticed the "bye-bye" valley girl thing. I am a little guilty of that myself, including saying "like" too much.
A couple of us around my office will joke around and do a prom queen wave with "bye-bye", or do the airline attendant "bye-bye", "bye-now", thing. (Maybe that's a SNL skit?).
Again, it could be a regional thing, because I have noticed that people from the Northern part of the country say "bye-now". (I am from Louisiana)
Sandra S.