No sound from standard Blu-rays on 4k player

No sound from standard Blu-rays on 4k player

I have a Panasonic 4k Blu-ray player, which works fine when playing 4k Blu-ray's, but when I play normal Blu-rays there is no sound, there is sound on the menu/title page but when the film plays there is no sound. Sound will only play if audio description is selected.

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