Custom music is music that is put in a game unofficially. Often custom music is made by hobby composers. Sometimes custom music is also a remix of official tracks.
History (about custom music in Super Mario Galaxy 2.5):
Custom music was just a dream. A dream that would never come true, because there wasn't a program that could convert between known audio formats and the Wii specific .ast format. Then the user "Kamimatt" released his astconverter and the 'composer' job was born. Several people applied for the 'composer' job. The board "Music" was created. Today in this board are about 30 Topics and 1200 Posts. Many custom music tracks were composed since then.
Contact:
You think you are a good composer and want to help us? Then come to our forum and register. There is a board which is named "Music". All things that are discussed there are all about custom music which will be in Super Mario Galaxy 2.5.
See also: Astsharp.exe