Complex sentence: Some of the films feature music said to be by a fictitious eighteenth-century Dutch composer named Van den Budenmayer; all music credited to Budenmayer is composed by Preisner.
Split into simpler sentences: Some of these movies make reference to and contain music by a fictitious eighteenth-century Dutch composer, Van den Budenmayer. In actuality, all music credited to Budenmayer within the context of the films is composed by Preisner.