Click onto an mp3 & then click onto File & Properties in your Windows menu & set the 'Opens with:' using the 'Change...' button to select WMP. This will set WMP as your default for opening all mp3 files.
Just as a side note:
Quintessential Player (awesome program by the way, and free) can play most anything WMP can play without the need to be constantly purchasing xtra codecs for particular types of mp3's or movies, etc. Just a thought!
I made a very simplified example here but obviously this was a text file & not an mp3. It's just to show what you need to do. Happy Listening!