If you're dealing with a musical in which you're trying to tell a story, it's got to sound like speech. At the same time it's got to be a song.
Musical comedies aren't written, they are rewritten.
Take a play that you like but you think is flawed, and see if you can improve it and turn it into a musical. Then make up your own story.
Deciding what is to be sung and what is not to be sung is really what writing a musical is about.