The utterance of "hmm" is key for listeners to understand that the speaker is currently engaged in thought; if the speaker thought silently instead, listeners may be unsure if the speaker had finished their utterance. "Um" and "er" are also used during thoughtful absorption; however, typically the extent of the absorption of thought is more limited since "um" and "er" are usually spoken mid-sentence and for shorter periods of time than "hmm". For this reason, thoughtful absorption is typically associated with the utterance of "hmm". "Huh" – the universal syllable
Research has shown that the word/syllable "huh" is perhaps the most recognized syllable throughout the world. It is an interrogative. This crosses geography, language, cultures and nationalities. See also
References
Further reading
Clark, HH. ; Fox Tree, JE. (May 2002). "Using uh and um in spontaneous speaking" (PDF). Cognition. 84 (1): 73–111. CiteSeerX 10.1.1.5.7958. doi:10.1016/S0010-0277(02)00017-3. PMID 12062148. S2CID 37642332. Archived from the original (PDF) on 2013-11-10. Corley, Martin; Stewart, Oliver W. (2008). "Hesitation Disfluencies in Spontaneous Speech: The Meaning of Um" (PDF). Language and Linguistics Compass. 2 (4): 589–602. doi:10.1111/j.1749-818X.2008.00068.x. hdl:20.500.11820/0e5f2f2f-7383-42c5-a7ba-63f2587ad877. ISSN 1749-818X. Daniel, Ari (5 Feb 2015). "Are we witnessing the death of uh? Um, maybe—and not just in English". Eklund, Robert (2004). Disfluency in Swedish human-human and human-machine travel booking dialogues (PDF) (Thesis). Linköping Studies in Science and Technology Dissertation No. 882 (Corrected ed.). Department of Computer and Information Science, Linköping University. ISBN 91-7373-966-9. ISSN 0345-7524. OCLC 940753621. Retrieved 11 August 2021. Erard, Michael (2008). Um...: Slips, Stumbles, and Verbal Blunders, and What They Mean. Anchor. ISBN 978-1-4000-9543-8.
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.