When somebody cannot answer you in a real sentence, but instead merely mumbles something you can't quite hear, and trails off; possibly chuckling as they do so. It is an unexplained phenomena that usually occurs under stress, or if that person is not comfortable speaking with particular people and is trying to get away.
Alyssa - "Hey Meg, how are you today?"
Meghan - "oh, wellidunnoijustsfjhsfjn..heh..heh heh..."
Alyssa (to another friend) - "looks like a case of inaudible mumbling."