Complex sentence: Sources close to the band claim that "Muzzle" was in fact due to be released as the final single, as is evidenced by the fact that a promotional single for the song was issued to radio stations worldwide.
Split into simpler sentences: Sources close to the band claimed that "Muzzle" was in fact due to be released as the final single. This can be evidenced in the fact that a promotional single for the song was issued to radio stations worldwide.