Complex sentence: Slade's soundtrack to the film was made up of Slade songs that had not yet been recorded or published, and the lyrics to many of them were re-worked to fit with the movie storyline.
Split into simpler sentences: Slade's soundtrack to the film was made up of Slade songs that had been kicking around but had not yet been recorded or published. The lyrics to many of the songs were re-worked to fit with the movie storyline.