If they are files you “ripped” from your own CDs and you used WMP’s default ripping settings, they are protected (DRM-crippled) Windows Media Audio files. While Sansas will play run-of-the-mill WMA files, they won’t play the protected variant. Protected WMA files (and any other unsupported format) produce exactly the results you described, by the way.
If this is the case, you’ll have to re-rip the files to unprotected WMA or another supported format.
