I have musepack plugin 0.94 for xmms from Frank Klemm's page and the sound is ok, but I still didn't figure out how to get tags. If I open the files that I encoded in WinXP, I get the filename in title tag, and all other tags appear to be empty. This happens with all mpc files I have, some have ID3 v1.1 and some have APE v2.0 tags. Is this a common problem or do I need to fix something?

BTW, is it possible to use replaygain for Ogg Vorbis in xmms?

This isn't made obvious, but the MusePack plugin for XMMS is quite old. It only supports ID3 and ID3v2 tags, no replaygain or clipping protection. According to Frank Klemm's website, they will be basing the XMMS plugin on the WinAmp plugin once it is complete. I find also that the player isn't very stable while using the mpc plugin, but that could just be me.

Anyhow, I have resorted to using mppdec for playing my music.

According to the dialog box that pops up when you configure musepack plugin, there IS replaygain and clipping prevention support. The sound is also much lower when I choose replaygain, so I guess it works.
Anyway, nice to hear that (someday) there will be better mpc plugin for xmms, hopefully as good as the one for winamp.