Thanks for the reply! I've re-downloaded Music Box today and it has installed just fine. Answering the Qs below in case it's useful…
> Did you have a previous version installed before or is this a fresh install?
This was a fresh install.
> Are you sure you’re uploading the .pdx file to the website and not some .zip for instance?
I was uploading the `.zip` of the game downloaded from itch.io (`music_box_1.1.pdx.zip`). Playdate's web sideloading UI asks for "a zipped .pdx file".