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1.
It has become common practice for some people to preface bye with a "mmm" particularly when ending a phone call. I have yet to hear this occur in vis-a-vis interactions.
I'll talk to you later. mmm bye. (click)
by James R Stevens March 07, 2008
 
2.
Noise made when ending a telephone conversation that is neither too abrupt nor too drawn out. If the person on the other end has a final thought the "mmm" part is a soft tone designed to let that person express the thought before the hard and final "bye"

As opposed to "kbye" which is too abrupt, or "Okay, talk to you later, goodbye, (OMG it's taking forever to hang up), click"
Customer "Okay I think that cleared up the problem."

Tech Support "Anything else I can help you with?"

Customer "I don't think so, thanks."

Tech "You're welcome, mmmbye."
by Accidental Linguist September 22, 2011
 
3.
It is the very hard to listen to way that a lot of folks will end a conversation. Instead of simply saying "bye" they have to say "mmmbye"... Through research it actually had been found that those folks have some kind of psychological problem.
Ill talk to you later! mmmbye.
by ProfEbonics June 01, 2010