Hi:

Every time I associate ALL audio formats to foobar, somehow after a a while, some file types, like mp3 and wma are strangely disassociated without any warning.
I also have WMP in my PC, but no formats are associated to it.
I had JetAudio until a week ago, but decided to try foobar2000, so back then, audio files were played only through JetAudio.

I appreciate your help! ;-)

The disassociation is the fault of your other programs. There should be an option within those programs to disable the hijacking.