It sounds like the primary audio track may have been broadcast 180 degress out of phase - that would account for the discreet stereo background information being clearly audible, while the monaural (information common to both tracks) sound "phase-cancelled" itself out. I heard this phenomenon recently on a cable commercial for the CD "Bee Gees #1's" - the stereo music was clear - the monaural announcer was virtually inaudible.