Lot of applications uses MIME type to recognize file types, for example on Linux many programs use shared-mime-info database:
http://freedesktop.org/wiki/Specifications/shared-mime-info-specAdding mo3 MIME type to the database would allow to associate a description and a default application to mo3 files.
I'm not sure if registration at IANA is required, maybe it is possible to use something like audio/x-mo3 (x means experimental), I can ask shared-mime-info Mailing-list about this.