Some have hypothesized that the time of an "uh" or "um" is used for the planning of future words; other researchers have suggested that they are actually to be understood as full-fledged function words rather than accidents, indicating a delay of variable time in which the speaker wishes to pause without voluntarily yielding control of the dialogue. There is some debate as to whether to consider them a form of noise or as a meaning-filled part of language. Speech disfluencies have also become important in recent years with the advent of speech-to-text programs and other attempts at enabling computers to make sense of human speech. "Hmm"
Hmm is an exclamation (an emphatic interjection) typically used to express reflection, uncertainty,  thoughtful absorption, or hesitation. Hmm is technically categorized as an interjection, like um, huh, ouch, erm, and wow. The first h-sound is a mimic for breathing out, and the second m-sound, since the mouth is closed, is representing that the person is not currently sure what to say ("erm" and "um" are used similarly). The pause filler indicates that the person is temporarily speechless, but still engaged in thought. The variety of tones, pitches, and lengths used add nuances in meaning. Etymology
The expression is used in many different languages, however the origin of "hmm" is difficult to find, mainly because "the word is so natural that it may have arisen at any time," as highlighted by linguist at the University of Minnesota and an expert on word origins, Anatoly Liberman.
Erard, Michael (January 3, 2004). "Just Like, Er, Words, Not, Um, Throwaways". The New York Times. Retrieved November 10, 2013. Ing, John (September 26, 2019). "Pause Fillers for Speaking". Retrieved May 3, 2020. Schuessler, Jennifer (November 9, 2013). "The Syllable That Everyone Understands". The New York Times. Retrieved November 9, 2013.