Transfer Grille Baffle
You could experiment with making the baffle out of a piece of a furnace filter. Filters allow the air to pass but would block much of the light and some of the sound. The more expensive type filters with corrugated construction and a HEPA rating would probably offer better sound and light filtering than the cheaper thin flat filters.

You will gave to study on your side just what type of baffle you could fit behind your grilles.