So. I don't know that I watch a lot of musicals. But then again, there are probably plenty of movies I watch that might be called musicals, even if my first thought is to put them in some other category. (Particularly Disney animated movies, and Muppet movies.) Anyway, to me the basic definition of a musical is a movie in which it sort of... doesn't make sense for the characters to be singing. (So if it's a movie about singers, it's not a musical, unless the characters sing not only at times that make sense, but also times that don't.) I may make exceptions for backstage musicals, though I'm less likely to do so for movies involving modern music such as rock bands.
See also musical web films
I have a separate page that lists pretty much any musicals I've ever seen, regardless of the medium (i.e., not just movies). That includes everything I've listed here, and a lot of other stuff. Originally, I also listed any musicals I haven't seen, but I later started a page here in the movies section specifically for musical films I want to see. It's possible there are some movies I don't include there that would qualify as musicals (per my definition), and it's also possible there are some I do include that I won't consider musicals, once I've seen them. But I'm not sure.