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NTSC audio is FM'ed on a 4.5MHz subcarrier, which subsequently modulates the transmitted carrier (channel) frequency. You won't be able to recover the audio with a TV unless you modulate it onto a carrier that the TV can tune. Well, I guess if you blasted the TV with 4.5MHz FM'ed audio at close range with high power, you MIGHT be able to recover the audio, but I doubt that that's what you have in mind.
I agree with Ron.This method maybe work with old TV-sets who contains in sound-IF channel filters with coils. Today all IF-stage built with piezo-resonators,i think can't pick-up signals from outside. (maybe with extreme high power)
Weren't the little black boxes we had to connect early computer games or computers to our TV sets more or less what ricardo might want? They essentially converted video and audio to TV channel 3 or 4. Not sure what they were called but if we knew the "name" it might be easier to find. I recall tossing several in the junk pile because I had no use.