Be sure they are 'real' audio files & not spammed viruses pretending to be audio files. Check their file sizes Virus thread: Typical virus spam file sizes - Make a note of them (click on blue link to see).

It might be a simple case of you needing an update of codecs to play such files. Have you tried playing them with VLC player? VLC is primarily a video player but can also play a high number of audio formats.