It would explain why we say "Mmmmm" not something higher pitched. Plus you can feel your tounge moving when you say "Mmmmm".
Any thoughts?
Thai say aroy ("tasty!") aroy mak ("very tasty!") or perhaps, if memory serves, gin dee ("good eatin'")!
'Mmm' is a very western sound I think. Probably not that historically established either, I wouldn't be surprised if it was a post broadcast media development. Remember how uptight the Victorians were. Perhaps it's French?
Or, less technically, when someone successfully hums a non-monotone tune- you have just heard what it means for them to assign a series of differing pitches to "m".
However, I suppose GP meant to write "we tend to say "Mmmm" with a specific, relatively low pitch", a statement with which I don't particularly agree. I would rather say it's done at an average conversational pitch, or even at a higher pitch since it often has a pitch accent conveying surprise. Which I believe is the main purpose of saying "mmm": communication, not vibrational tongue massage. Consider that when you say "mmm", you voluntarily choose a pitch intonation according to what you're trying to convey, ie. how pleasantly surprised you are.