Try playing the files with VLC player. It's a free download & has codecs installed to play most file types. It's a very reliable player.
VLC media player

Do CDs play without a problem? What about songs that you downloaded a long time ago? Do the file sizes (of songs that won't play) still look right?

If the files won't play using VLC, update your AV & scan your computer. If you don't have an AV, these are free (make sure to update before you scan).
avast!

AVG free

Avira Antivir

Or, there are links to online scans in the thread below.
WARNING: Viruses on network you should be aware of!