A little neuro here: while the "memory" for what "words" "mean" tends to be "represented" in the left hemisphere, music tends to be predominantly right-hemisphere. It is well-documented (short for I can't remember any specifics) that left-hemisphere strokes, which often seem to eliminate the normal use of language, often has no (or a markedly reduced) effect on the ability to sing songs. In fact, the more lyrics you know, the less likely you are to be debilitated by a left-side stroke, as even if you can't remember the word Spoon when wanting a spoon, you might be able to sing That Lovin' Spoonful or something and still get your message across. Of course, the more languages you speak and the more X chromosomes you have (up to the normal complement), the less lateralized your brain is to start with, diminishing any "savings" effect of knowing songs.

My field was cognitive, so you'd think I'd know more about dementia, but I don't. I will, however, ask around.