呣 with a falling tone (m̀) expresses agreement or acknowledgment. With a rising tone (ḿ), it expresses a question. Similar to English 'mm-hmm' and 'hm?'
Left mouth radical signalling a vocal sound — 呣 is the written form of the closed-mouth grunt mm, the nasal hum you make when answering 'mhm' or pondering. The radical is what turns a sound character into an interjection rather than a noun.
Right 母 supplies the sound, mǔ approximating the closed-mouth m̀ hum. Pure phonetic borrowing — the 'mother' meaning of 母 is irrelevant here. It was chosen because the initial m- and nasal quality matched the bilabial grunt being transcribed.
