Thanks for reporting that Will, I've changed to .mp3, but unfortunately in my testing mp3 is not working on firefox. Firefox claims to support it but in my browser is:
a) not producing sound
b) producing errors in console. (format not supported)
However the game is initializing and playing, so I have converted it to mp3. (It's an improvement: one browser still not supported but at least it's running)
Firefox claims it requires the system to support mp3 decoding and, well, my laptop can play mp3's just find with vlc and other media players, so I'm not sure what firefox is on about.
I'd be interested to know if this solves @ayrin's problem as well.