Reader Ted wrote in with this tip, which solved his problem of playing ripped music files in either Windows Media Player or Media Monkey.
Step 1: Uninstall Windows Media Player
Open up Control Panel and type “windows features” into the search box, and then click on Turn Windows features on or off.
Go down to Media Features –> Windows Media Player
Step 2: Reboot
That is all.
Step 3: Turn Windows Media Player Back On
Head back into Windows Features on or off, and check the box again.
At this point hopefully your problem should be solved.