If someone says mmmmmmmm is it a good sound or should you be worried

If you're on the table with an apple in your mouth and they're holding a meat fork and carving knife, I'd worry. Otherwise, mmmm is usually all good

Thar depends if I am lost in the Appalachian mountains or not.

Depends on who, and the context

I will have to think about that!

I think usually when I say “mmm” I’m thinking something over

It depends on what they're saying "mmm" to

A submissive bitch wants to suck is what that means