wmp only? Do you mean the wmp icon or only wma files?
If wmp icon, then sounds like your wmp has inadvertently taken over as the default audio program, to learn how to change this back, see Setting default audio program. In a nutshell, right-click any .mp3 file on your computer, choose Open with .... choose iTunes, & check the option to always open with.

If only wma files, then check your keyword filter & be sure mp3 is not listed.