It appears to already have the MIME types in the .desktop file...
probably isn't running the desktop-file-utils progs during install.

redhat-audio-player.desktop
...
MimeType=audio/x-mp3;audio/x-mod;audio/x-wav;audio/x-mpegurl;
audio/mpegurl;audio/x-scpls;application/x-ogg;

Since XMMS isn't part of our default install we should transition the
name over to Name=XMMS Audio Player, however we don't need translation
hunting us down for this one so it can probably wait.

hmmm i think the mime type list in the xmms desktop file might have a
syntax error. You might have to add application/ogg;  to get it to
show up in nautilus for oggs. 

i just installed xmms from the development tree
and the postinstall script does the call to 
update-desktop-database /usr/share/applications
and I see the Mime-type definition
but....
xmms is not showing up in the right click menu in nautilus for ogg
files as a choice. Nor does it show up in the "Open With" tab of the
file propreties dialog.
but....
It does show up in the wav files as a choice.

Here is a comparison between rhythmbox and xmms Mime types, you might
consider adding more than the application/ogg. I'm pretty sure xmms
can do flac as well even if it does take a plugin.

/usr/share/applications/rhythmbox.desktop:
MimeType=application/x-ogg;application/ogg;audio/x-mp3;
audio/x-scpls;audio/x-mp3;audio/x-mpeg;audio/mpeg;
audio/x-mpegurl;application/x-flac

/usr/share/applications/redhat-audio-player.desktop
MimeType=audio/x-mp3;audio/x-mod;audio/x-wav;
audio/x-mpegurl;audio/mpegurl;audio/x-scpls;application/x-ogg;

-jef

There were actually two bugs here:
1) missing application/ogg from the redhat-audio-player.desktop
2) xmms calling update-desktop-database /usr/share/applications
instead of update-desktop-database /usr/share/desktop-menu-patches

Both are fixed.