Thank you AllbadOtk for asking. I will try to explain the most common noises off the top of my head.

As you stated "Mhm" is an example of humming in agreement.

"Hm?" Often is used in a sort of questioning manner.

Groans are two types. There is the "I am in pain" groan and the "I am exhausted/exasperated/tired of <insert thing>" and they have distinct noises. The second groan is often sounded with "ughhhhhh" while the first groan often has "ugh". The difference is the second groan is significantly longer and usually followed with some sort of closing sigh.

speaking of Sigh. Sighs have a few different forms. They can be sighs of relief, sighs of release, or sighs of exhaustion. Sighs of relief are characterized with a smile at the end of them. Sighs of release often have the releasing person seem less tense. Sighs of exhaustion are the most common sigh, and tend to be your average sigh. There is also the sigh of acceptance, where something happens and someone does something and the sigher just sighs, accepting their fate.

Moans tend to be used only in sinfics.

Huffs/Pants are heavier breathing compared to normal. Panting is usually used after someone is exhausted physically. Huffs are like if Pants and Sighs had a baby and that baby was used exclusively in exasperation.

Hums have many forms too. Humming in thought, humming in song, humming in agreement or disagreement, or just humming to question.

"Hmmmmmm." Is often used when someone is thinking but needs to keep the conversation on them while they collect their thoughts.

"Hm-hm-hm-hm-hmm-hmm-hmm" (Think His Theme, or humming along to P!ATD or FOB or any singer.) This is pretty straightforward.

"Mhm" "Mm-mm" "na-uh" etc. Mhm is agreement. Mm-mm can be neutral or disagreement or even agreement. Na-uh is disagreement. The way to tell is the ending noise. Mhm ends on a higher note than it begins, so it sounds agreeable. Na-uh ends on a lower note than it begins, so it sounds disagreeable. Mm-mm can go any way depending on the ending noise.

"Hm?" ends higher than it's longer thought cousin, and that higher note at the end is higher than even agreement, which often leads to sounding like a question.

(Many of these sounds rely on tone inflection, which is basically how it is tonally spoken. In other languages, this inflection is different. In English, the general consensus is higher at the end generally means question, while neutral or lower indicates a period. Shouting would be indicated with '!' or CAPS LOCK.)

One final thing that came up while writing this chapter.

Onomonopieas.

"Buzz" "Hiss" "Bark" "Woof" "Meow" "Bang" "Crack"

What do all these sounds have in common?

They are words which sound like the sound they describe when you say them. Don't believe me. Try some out (Notable ones would be Buzz, Woof, and Bang).

As you noticed, when you say these words, they produce the sound they try to describe. That is Onomonopiea. Honestly probably the easiest rule/grammar/writing/english thing I will run into.