There are actually many reasons why an mp3 isn't working.
1. You didn't download or rip the file properly or the file is corrupted.In either case you may need to re-download (from a more trusted source if you're a pirate) or clean the cd and rip again. I've had this problem on a few albums where the cd is messed up and can't rip properly. I've also downloaded from untrusted sources (bought a cd, broke it, downloaded a pirated copy because I already own the cd) who's rips are deplorable.
2. Your codec is corrupted or not there. Go online and find an mp3 codec pack. This will help you play them accordingly.
3. Your speakers are messed up. Try playing the files on another medium, such an mp3 player, someone elses computer, burn it onto a cd and listen to it in your favorite disk playing gaming console. If it's your speakers re-download the driver (which you can find on the manufacturer's website). If that doesn't work your speakers are broken.
4. It could be a problem with WMP itself. That program has always had a large number of bugs and problems (not Microsoft's best program). Uninstall, re-download and reinstall. You can find the program on Microsoft's website.
5. Last and certainly not least, it's another unexplored, unknown problem with the programming or music. If any of the previous solutions don't work then I can't think of another reason there could be.
Hope this helps.