Hi LOTR fans,
Back when the ROTK hit theaters there was some discussion about how the film makers produced the high pitched scream made by the black riders.
Watch the producer-director commentary on the "Fellowship" special edition and hear it straight from the screamer: it was none other than co-producer Fran Walsh with a sore throat. She discovered she could make this horrible sound and Peter Jackson decided to use it. She explains how she made a whole set of screams until she was practically unable to speak. The sounds were then "doctored" in the digital audio realm and used for the black riders.
There's a lot of amazing and amusing comments about the production, such as actor Hugo Weaving (Elrond) claiming he only does trilogies now, because he also played the Smith character in The Matrix trilogy. Fun stuff!