Why Isn't Audio Playing in Firefox?
Some codecs will play in some browsers, but others not. Firefox might not be able to play audio if the file is saved with PCM audio. Try AAC instead.
For Mac users: Select your file and press CMD + I to quickly view information about the codec.
For PC users: Right-click on your file, select 'Properties,' and navigate to the 'Details' tab to find codec information.
Check out Handbrake or MPEG Streamclip for two free and easy to use video converters. Apple's Compressor is also recommended.