You're right, Bob, that the musical term [adjective or adverb] is Italian, and that the English noun [for "strong point"] should be pronounced "forte" if it is derived from French. But if French has no such noun and the adjective "forte" exists [with exactly the same spelling] in both French and Italian, then why assume the English noun is derived from French, rather than Italian? [Of course, etymologists probably have ways of tracing the term's history, but couldn't the term have evolved just as easily from Italian as from French?]

In either case, you're correct that usage has it overwhelmingly as "for-tay." If you say, e.g., "Folk music is my "fort," people look at you like you're an ignoramus!"

Sigh...