in other words, "Mmmm" used as a tone of appreciation or approbation, NOT of deliberation as in "Mmmm, let me think about that"
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You can say:
Note that the actual pronunciations of these う/ふ/む may be different from what you already know. These interjections are often pronounced just like English "mmm" or "hmmm". See: IPA for ふーん/んふふ (pronounced with your mouth closed) and Difference between うーん and ううん