My views on the film's baffling ending.
I'll give my own interpretation, which may very well be wrong. I believe it to be a movie within a movie within yet another movie in which the unsuspected sound engineer is the star of a psychological cinematic documentary experience without his knowledge or consent. To my mind Gilderoy was an unaware participant in a psychological cinematic documentary experience at which the moviemakers and the crew's objective was to push Gilderoy towards a breaking point and film that gradual loss of control, which was essentially the movie we have watched.
Why have I made that reasoning? First of all because they chose a sound engeneer that sounded children's programs and pastoral documentaries. They believed that by placing him in an element with such a violent subtext as is an italian psycho-sexual and extremly violent supernatural horror movie they hoped that he would come to a breaking point and somewhat lose his mind, thus exploring the fragility of the humam mind and film it too. They added to the tension by hinting that the all production was shady (they made constant excuses not to reenburse his flight)and something dark and seedy was taking place in the studio, whose atmosphere was clasutrophobic and scary in itself. As if that wasn't enough they made one of the actresess pretend that she was abused.
The third thing that made me reason as such is the fact that the last minutes of the movie are completly dubbed in Italian, including Gilderoy's voice and the fact that right at the end he plays the supposed footage of the movie he made the sound for and there is nothing there, just the sound and no image watshoever.
If I am right and that is what the end signifies than you have a triple exercise in metaficiton in which you have a movie within a movie within a movie within a movie; that is, the movie Gilderoy went to make the sound for; the one who is being constructed at Gilderoy's unknowing expense - a kind of reality documentary that films his gradua reach of a braking point and finally the one we have just watched, directed by Peter Strickland.
That's how I view it anyway.