well oss is deprecated by alsa. and esd is just a sound daemon used by gnome (and you missed artsd used by kde). a sound daemon is kinda like a traffic cop for the sound system.
the musepack development team wrote and maintains "libmpcdec" which is a library decoder for the musepack format. a sound program (player) or even the sound system itself (gstreamer) would have support for musepack as they utilize the library decoder (libmpcdec) currently on the system.
the short version is "already covered"
what makes you ask?
later