[iTunes] iTunes rejecting mp3's

[henri]

I've run utilities, repaired permissions, etc. Does not solve this 
recent problem:

I've bought music in places other than iTunes, plain MP3 files w/o 
any known DRM protection.
I've brought them into iTunes w/o problem until recently. Now when I 
use the Add to Library command, nothing happens. Import, nothing 
happens. Drag the files to iTunes, nothing happens. I double click 
and Quicktime plays the songs perfectly. I cannot bring these files 
into iTunes. Can anyone tell me what's wrong and how to correct it?

Intel iMac 2.4gh , 10.55, everything up to date