Munya munya is the term used in Japanese to describe the sound and sensation of mumbling. It’s a great word. I love the way it rolls… or, sort of, clumps… off the tongue. As an example:
Zen zen David Duchovny no munya munya wo wakaranai.
(Is it still funny to joke about how David Duchovny talks?)
Munya munya is part of a whole class (no, not that kind of class) of Japanese onomatopoeia words (hardest task today: figuring out how to spell onomatopoeia), including both words that represent the sound of something, like munya munya, and words that represent the feeling of something, like peko peko (the feeling of being really hungry).
To date, Japanese is the only language I’ve encountered that actually has a “sound of silence.”