How to fix Quicktime Plugin error in Firefox

I've been looking everywhere and the only place I found the answer was in a cached website that is no longer available. And, I know many people have this problem; so I figure it is worthwhile to repost it.

In Firefox, if your browser shows the "Additional Plugin Required" error on pages that have sound files, and it prompts you to install Quick Time, but you already HAVE Quick Time, here's how to fix it.

Open the Quick Time Player.

Click Edit->Preferences-> Quick Time Preferences
In the drop-down box, select Browser Plugin
Click the Mime Settings Button
Under Audio, select WAVE AIF and MIDI
Under MP3, select both checkboxes.

Save the changes and restart firefox. Problem fixed.