Never heard of this before.
When you play one of these "without audio" songs, what happens? Be very specific, including what does the Playlist window
Dur(ation) field show for thes files, and what is the actual playing time when you play one?
The more you can tell us the sooner we can diagnose what is happening here.
It is a unique problem.... 
Create a Playlist with some of the "files with audio" and some "files without audio". Play some of each in Auto Play mode (click the Auto -vs- Man button on the Playlist screen) and insure you hear the audio in the first but not the second. I'm wondering if your audio somehow got disabled?
PS: I deleted your your email as this is a public Forum. Those nasty Spammers have robot scripts that crawl through public Forums looking for email addresses to sell. Thank "George" for warning me. I hope I deleted it before you got spammed.
This Forum PROTECTS your email address, yet allows us all to communicate with you and you with us.
Please do not post your email again.