If mum has old-style fillings in her teeth, that has been known to be enough for some with those fillings to be picking up radio waves, where they hear, strongly or remotely, music or people talking.
Could be some element / elements in the house where something is audible via those elements acting as radio receivers.
I could barely hear anything, and would add that if you turn off all electricity, that would eliminate subtle harmonics from refrigerator motors, etc. which can often trigger these heard 'musical' things which are audible, 'really in the room,' and recordable.
What it is not is extraterrestrial, spooks, 'music from the other side,' etc. (Youtube is littered with 'music from the angels of the heavenly choirs,' or spirits. Some of those are transparent straight-ahead hoaxes, others more than likely from coincidental confluences of electronically generated sound waves.)
One of those I heard was 'music' picked up over the speaker system in a church, which I've experienced in a school's chapel, but that was not music via radio waves, but city police radio. Passing taxis and police cars triggered a radio doorbell a friend of mine had installed!
The lists of 'what is making that sound' in the realms of plausible (and actual) cause and effect are near endless, and should not be overlooked or crossed off before they have been exhaustively investigated prior jumping to favor the paranormal.
If this mysterious music is not really upsetting your mum, then leave it unexplained. It then has her not alone in her house, gives her something to talk about which brings attention to her when she talks with friends and acquaintances, makes her 'of interest,' -- in which case, leave it undone / unsolved and let her have it as it is