I'm 25, and I don't know much about the neurological in this example, but when I'm really high, I sometimes think I hear music playing (claasical or rock both of which I don't listen to on my own) when it's actually a wind breeze or pipes in the building. Deciphering the source of the sound and then remarking that the music oddly resembles said sound makes me realize that my brain was filling the gap of something faintly audible.

It is incredibly interesting how sometimes I could hear extremely well crafted tracks. Although I have no musical training at all, the subconscious seems super creative but also very, very convincing, I have zero doubt your grandmother 110% believed what she was saying having been caught off guard myself truly thinking there's no way I'm not hearing a neighbor's music and that it's actually me taking a random sound pattern I was ignoring and seemingly turning it into "proper music".