For what it's worth, I went ahead and gave one of my DMG-001's (the noisest one) a cap kit and, to my surprise, it didn't seem to improve things music. That said, I listened to your recording and I don't hear anything abnormal except that noisy bit right before the square. I think that might not be much to worry about.
By contrast, my GameBoy emits a high pitched whine that is independent of the volume knob and it drives me nuts! My other DMG has that same high pitched noise but it's much much quieter. It's a bummer since that's also my GB with the backlight mod.
In any case, I would try yours out with LSDJ, which has an option to control power save mode and can give you a better idea of how noisy things are. In lieu of that, if you have a game that has spare music or music at a lower volume, you can better judge things.
