Probably either corrupt fakes or viruses.
1.
mp3 Bit-Rates Versus File Sizes (click on link)
2. See
WARNING: Viruses on network you should be aware of! (click on blue link) 2. (n) & (o) __ (these viruses only apply to windows pc users so will not affect the OSX system but will be non use-able files.)
3. Check their file size within iTunes, if less than 2,000 KB or 2 MB then they are probably not complete files or some type of fakes such as pc windows viruses. They might even be empty preview files if you tried to preview the audio file before the file had downloaded any sufficient file information.