Actually, I HAVE read it, and it's a subject that came up regularly in Navy Rights and Responsibilities (a 1 week class that everyone had to take once a year in the Navy).
And, while it doesn't specifically mention Japanese metal, it doesn't specifically mention Christianity either.
Here..................read it for yourself.......................
The First Amendment allows citizens to express and to be exposed to a wide range of opinions and views. It was intended to ensure a free exchange of ideas.
www.annenbergclassroom.org
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
And, because it's not specific, there is room in there for political activists and Jehova's witnesses (like there is for Christians), as well as allowances for Japanese metal, because nothing is really specified. Japanese metal could be considered free speech.