This happens a lot if you installed or upgraded another media player, it wants to grab all the file types to play on itself.

Somewhere in the iTunes settings (Edit > Preferences > Advanced) should be a button "Warn me if iTunes is not default player for media types" or something like that. Check that box, click "OK" then restart iTunes. It should give you the chance to reset file types to default to iTunes.

This should fix your problem. If not, then the Apple forums might be able to help.

I agree the file associations got changed. I had been able to load MP-3 and MP-4 from laptop to IPhone via ITunes. And I do remember doing something that changed the file associations. But, the files were still MP-3's and MP-4's just not compatible with my IPhone. There seems to be a difference between MP-4 files. I would like to know how to tell the difference and which will work with IPhone.

Assuming you are on a recent version of iTunes....

Connect your device to your PC running iTunes.
In iTunes, go to the Summary tab.
Scroll down to Options.
Check the "Manually Manage Music and Videos" option.
Click "Apply" at the bottom of the section.