Ok I understand what is going on, but I have no idea why. Immediately after the game loads (unity bar), you must click on the screen, and the music starts. It seems that the window is seen as inactive or something like that. It seems that clicking on the window allows the reproduction of the audio to the browser. Honestly, I never saw anything like that. I guess it is due to FMOD, it is the first time I use 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