Sadly Firefox doesn't support the WebAudio API, only an older audio API. The WebAudio API was designed primarily by Google and is very nice. It's only supported by Chrome AFAIK. Last year at Google IO they had a talk about it. We use the API in our game, and it works great.
Nightly builds of Firefox are starting to get support for the Web Audio API, hidden behind a pref 'media.webaudio.enabled', which can be set in 'about:config'. At some point when it's considered more useable someone will blog about it and the state it's in.
It's certainly possible to play sounds in all modern browsers. It may be a bug in the game. Or they simply decided not to support all the APIs. We did that because it doesn't make sense to support two different sound APIs when in a few months there will be one as a W3C standard supported by the two major browser engines (screw IE, it doesn't even do WebGL).
