Go into Tools>Options in Firefox and then go to the Content tab and click manage (near the bottom). Make sure that all the file types listed have a program / plug in to open them. When you first install Firefox you need to go round getting all the plugins installed.

If they are all ok then I would uninstall all the programs that you have installed recently that affect your audio, any players etc.

Finally you can install all the latest audio codecs from Microsofts site here