Today I went on a head to head music-quality (through headphones) comparison between N9 & N900.

I have both phones in high esteem when it comes to music (among other things of course) but I have to give it to the N900.

A tad louder, more bass, and wider "audio spectrum" (if I can call it that).
N900 also takes less time switching between songs. It is noticable faster. It is almost a good 0.5'' faster. Almost gapless.

N900 takes... 0.6'' and N9 takes around 1.1''


*artists tested: Daft Punk, The Prodigy, Metallica : D