My sister in law’s grandmother had been demented and circulating among the daughter’s houses since SiL’s childhood; she died last year, some 32 years after the rotation started. She hadn’t talked for over 25 years at that point, but almost until the end she sang, church songs.
At one point SiL (who was an MD by that time) realized that her grandma was actually using the songs to communicate, that the song would change depending on her mood and whether she wanted something specific; most of the time she sort of hummed random bits, but if she started singing louder, longer bits, it meant she wanted something (none of the daughters qualifies as “the brightest bulb on the tree” unless the rest of the tree is burnt out, they hadn’t realized this in some 20 years). My brother had the theory that some songs meant specific needs and that it wasn’t necessarily linked to the words of the song, but for example a bouncy song would be more likely to mean “please walk me for a bit” than a slow one.