I just tried searching for "Plan 9 Misty Mountains"... GAF was the second result.
"Plan 9 band" came up with one band in Rhode Island, US and another in Germany. Listened to samples from both and neither really sounded like they would have created the song. There were no announcements on the site of either band.
Going to get more creative in a sec.
edit:
Old interview. They are not new to the movies. Did not know this, and I feel like I should have.
I am quite comfortable with this now knowing what part they had in LotR.