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The local oscillator is most likely off frequency.
Unless someone in the past has monkeyed with the video or audio IF, the local oscillator could be set to the other side of the channel.
If you are using cheap consumer channel 4 RF modulator, it produces a double sideband am video signal. It is conceivable the video carrier is on the opposite slope of the IF response so that the picture looks good but the audio would be absent.
Try adjusting the tuner across the channel and ensure the video carrier is on the correct side of the Video IF.
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