FileInfo.com - The File Extensions Resource

.MUS File Extension

File Type 1	Finale Notation File Format (Most common)
File Format	File
Common?	No
File Description	Music notation format used by Finale and other MakeMusic notation programs; saves data in MIDI format, similar to a .MID file; also contains additional information that can only be read by MakeMusic software. MUS files contain music notation information, but do not store actual audio data.
Program(s) that open mus files	Mac / Windows: MakeMusic Finale; MakeMusic Finale Allegro; MakeMusic Finale PrintMusic; MakeMusic Finale SongWriter; MakeMusic SmartMusic

File Type 2	Playstation 2 Music File
File Format	Standard File
Common?	No
File Description	Contains music data referenced by a PlayStation 2 (PS2) video game; often used by games developed by Electronic Arts (EA); typically used for background music that is played during gameplay.
Program(s) that open mus files	Windows: MFAudio

Information from FileInfo.com - The File Extensions Resource.
http://www.fileinfo.com/extension/mus