If the Firefox browser asks you to install the QuickTime plug-in to play an MP3 audio file or MPEG movie, and you already have the QuickTime plug-in, then you need to adjust your QuickTime configuration settings. Follow these steps to resolve the problem (on Windows; similar steps work on other systems):

1. Start Menu -> Control Panel -> QuickTime

2. Select the "Browser" tab

3. Click the "MIME Settings" button

4. Make sure "MPEG" and "MP3" are selected

5. Make sure you do NOT select "Flash media" under "Miscellaneous," this will interfere with the normal Flash player

6. Click "OK"

7. Click "OK" again

8. You should be done. Restart your web browser and access the page with the embedded sound or video file again.