I'm playing in Safari on MacOS, which does not have .ogg support and thus tries to get .m4a files for the music; I'm assuming this is something the engine automatically does for cross-browser support?
I hadn't actually noticed there was meant to be music until until I reached the full choir, so I checked the in-browser Developer Tools and there are messages appearing in the browser console saying the .m4a files yield a 404:
https://v6p9d9t4.ssl.hwcdn.net/html/287757/media/k...
https://v6p9d9t4.ssl.hwcdn.net/html/287757/media/f...
(etc)