I just visited the dentist and it occurred to me that even with various pieces of dental equipment in my mouth, I could still answer a question with uh-huh or uh-uh.

It may not be the particular vocalizations that matter so much as that in sating uh-huh the pitch rises on the second syllable, and in saying uh-uh the pitch stays the same or maybe goes down a little. You could achieve the same effect by playing two notes on a musical instrument.