Mom is constantly humming. Why is she doing this?

I'm only 61 and I hum or sing(though I really can't sing)pretty much all day. I usually wake up with a song in my mind and it goes from there. I even hum all through the stores when I'm shopping. I"ve had several people over the years stop to tell me that my humming meant that I must be a happy person. I've honestly never thought much about it, until you questioned it about your mom. Perhaps like me, and what graygrammie said below, she just "has a song in her heart."

Perhaps she always has a song in her heart?

I have been "accused" of whistling too much, but it truly is because there is always a song in my heart. Plus our parrot whistles a few notes and I take it from there. Husband will ask why I whistling or humming something and I'll say, "Hey, Conner (the parrot) started it. I had to finish it!"

If she is calm, this is just her way of self-soothing or form of singing. If the volume is a problem, then have her hearing checked.

My Husband did that for about the last 9 years of his life. I could not ask him about it as he was non verbal at the time.
Drove me up the wall for a while. Then It dawned on me...I'm gonna miss that when he's gone. Did not bother me as much after that thought.
And I do miss it.

Noises are a way to calm themselves. If she has a problem with tinnitus her humming might be a way to cancel out the noises she hears in her head. Sort of like a White Noise machine.

Can you wear headphones or earbuds? That might help a bit.

Does she have Dementia? If she starts getting louder and louder, I would call her doctor and see if she has anxiety. She will need to be medicated because not good for her to be anxious all the time.