Thank you very much for the help so far!
Aiwendil's essay is indeed very helpful, an excellent summary, and the two to which he links are very good as well. I'm encouraged to carry on now; I had been thinking about the topic for some time, but couldn't find enough material to use for the research. (I hope to present the topic as a lecture at next year's Tolkien Seminar here in Germany.)
Thanks,
Rumil,
Legate, and
mark for the references; it does help a lot when I can narrow down the source material reading - combing through all of Tolkien's books, including HoME, is a daunting task and more than I can handle, what with "many duties that I did not neglect, and many other interests... that often absorbed me."
Bb, there's only one answer to your contribution:
Well, maybe two more:
Actually, I do plan to mention the movie usage of instruments as a brief sideline - how about the percussive use of a whip in the animated RotK's song "Where there's a whip, there's a way"?! (Sorry, no smiley allowance left to use here...)