No, it currently does not. That's not something I ever plan on supporting due to aspects I explained previously (there isn't a one-to-one correlation of songs to files) and the fact that if you renamed a file, for example, MobileSheets wouldn't know you did that, and it would think you deleted the old song and created a new one, and you'd instantly lose all your annotations, bookmarks, smart buttons, link points, etc. You can certainly just batch import the same Dropbox directory multiple times to pull in new files though (some users like to operate this way). You'd have to manually delete songs in MobileSheets though.
Mike
Mike