Yea it's a very odd bug I've done a lot of webgl-ing and i've never come across anything quite like it
Viewing post in CountdownCastle jam comments
I read FMOD documentation, and found out that they require a player interaction to load the banks into the browser. Not sure why they do. I am working on adding one more screen in the WebGL version of the game, so that the player is forced to click to start the game. It's very annoying, but at least I am sure people will hear the music :D