You thought wrong. The sound of "hmm" is more inquisitive in verbal use, and usually means that the person didn't hear or understand what you were saying, especially if they stress the ending 'm'. If they stress more of the beginning 'hm', that's just more of an internal thought to them verbalized.
Still the sound you are talking about is mmhmm not mhm
Edit: I'm sure text shorthand would make it mhm along with ur though haha