Is /music/ using an index.md page? You need to rename it to _index.md for the children pages to be navigable, otherwise they will be treated as page resources only. See Content Organization | Hugo
If that does not solve your problem, you need to share a link to your site code repo so we can have a look. See Requesting Help
Thanks! I renamed index.html to _index.html, and then the Content Organization page helped me to understand that instead of a content/music/foobar directory I could have a foobar.md file in the content/music directory, and that now shows up as http://localhost:1313/music/foobar/, which was my original goal.
http://localhost:1313/music is now showing the menu and sidebar but it is not showing the the index.md content that is now in _index.md. How can I do that?