As Lykofos mentioned, there are addons that will play music for you and act as an mp3 player. I can't think of the name of one off the top of my head, however.
Also, while outside programs cannot interact with WoW, some do have an
overlay for the screen. WinAmp, for example, has a setting to pop up a box in the corner with whatever new song it's playing.
/edit: to be able to control my music from "within" WoW, I <333 my media keyboard. Let's me skip ahead, pause, adjust volume, etc.