If you're attempting to play sounds using the new HTML5 <audio> tag (or using the .play() Javascript API method), and it isn't working in Firefox, it's because Firefox won't play MP3s or WAVs. You'll have to convert your sound files to OGG files.

if you're a Mac user, download the XiphQT Quicktime component. It will let you export sound files as .ogg files using Quicktime Player 7 (located in your Utilities folder).