This constant humming and mumbling, normal for her declining state? Or is this some self-soothing behavior?

If you look up Teepa Snow she has some great info on this behavior. She is a WONDERFUL resource!

Yes. My Mom hummed but it got louder and louder and was more an anxiety thing. If she gets to this point, she will need meds.

Sounds very normal behavior for a person with dementia. My grandmother mumbled to herself. I tried to make out if some of it was actually words...? But it didn't seem to be words, but more like it was repetitive gibberish. It was definitely a self soothing behavior, for her.

I think you're right about the behaviors being 'self-soothing'. My nephew who had autism did the same thing for a while, just to cope with his environment & his sensory sensitivity.

I'm assuming your mom has Alzheimer's in the advanced stages. The reasoning part of her brain is gone. (They actually loose brain matter. It shows up as empty space in the cranium.)

Even though I have not witnessed my mom doing that, she will sometimes talk to herself when she's disgusted. All of these behaviors are "normal" for dementias. Everyone manifests the disease differently. Sadly, they have left this world and are in their own existence.

Most likely the humming is part of the decline. It may soothe her or when she mumbles she may think she is communicating normally. My mom can still carry on a decent conversation if it is simple, but any complications, additions, switching of topic, whatever, she is lost and gets agitated. The fewer things, especially noises, happening around her, the better she feels.