Complex sentence: Not finding one, they found a guitarist, Sonny Moore, upon hearing Sonny singing'Featuring Some Of Your Favourite Words' in the studio, it was decided that Sonny Moore would be the Lead-vocalist.
Split into simpler sentences: Not finding one, they found a guitarist, Sonny Moore. Upon hearing Sonny singing'Featuring Some Of Your Favourite Words' in the studio, however, it was decided that Sonny Moore would be the Lead-vocalist.