Hmm, I would arrive at the opposite definition of "information density" than you, and I'm guessing the GP does, too.

Your definition is "musical notes played per surface unit", but I think that's inverted. Taking a musical note here to mean a fixed note length, more surface per note allows for additional expressiveness. You can even "fit more notes in" if the notes are a shorter duration.

A time span on the outer part of the record has more groove length dedicated to the same amount of playing time, so it has a higher information density in my book. I guess my definition for density would be groove length divided by playing time.