I believe XMMS was available for the N810 under Maemo 4/Diablo, but I don't know how complete the port was. If you know the basics of compiling under Maemo, you could try simply recompiling the package for Diablo, and see if it works (no guarantee, but usually worth checking).
As for Amarok, I doubt the UI would be very suitable for the N900 form-factor, but there has been a fair bit of interest in porting KDE software to the N900 (look up posts about the recent "Tokamak" meeting of the KDE Plasma team, for example), so I suppose it's not out of the question. Maybe poke someone on IRC (#amarok on Freenode), and ask if they've thought about that at all. As I said, the UI isn't really suited, but if their code is such that large parts could be reused (as in the Maemo KOffice port), maybe someone would be interested.
In the mean time, Canola has been sitting in Extras-devel for ages, so I'm not sure what state it's in, but it was quite nice on the N810. The usual caveats apply, but if you're feeling adventurous, it might be worth checking out. (You probably should do a quick search to see if anyone's said exactly why it hasn't been promoted to -testing or Extras yet - if it's horribly broken, you might want to know ahead of time :P)