I'm finding this thread very interesting, though it seems to be wandering far and wide!
BTW, "Guest" who doth keep proclaiming lack of interest in the topic and questioning its fit to "folk music,"
there are songs most definitely adopted as "folk music" (not to mention other genres such as jazz, blues) that come from opera. such as the Gershwin/Gershwin/Heyward aria "Summertime," from "Porgy And Bess."
Which brings me to the question of what distinguishes a "musical" from an "opera." My understanding, simplistic as it may be, is that in an "opera," all or nearly all the lines are SUNG, whereas in a "musical, " spoken dialogue is interrupted from time to time (sometimes quite unnaturally) by the actors bursting into song and maybe dance as well.
So the original "Porgy And Bess" (as, I understood it, George Gershwin construed it), as well as Lloyd-Webber's "Cats," the Broadway version of "Les MisÚrables," and the show "Miss Saigon," are really operas (even though they're - shudder! - in English and don't sound like "classical music." Almost all other theatre productions and movies I can think of where there are a lot of songs sung/danced by the actors are "musicals," complete with a lot of spoken dialogue.