I just finished building a new home and as part of the Energy Star rating transfer grilles were installed above each of the bedroom doors to allow air exchange when the doors are closed. The problem is that sound (and a small amount of light) pass freely through the vents negating all privacy of closing the door. I've read that I should install a baffle into the vents to allow air to pass through but deaden sound. I can't find any details for where I can buy this or what kind of materials I can use to construct it. Any advice would be greatly appreciated.
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.
Building Science suggest simple sheet metal baffles. They'll certainly be effective at blocking the light. It's probably not the most effective soundproofing, but it may be sufficient to bring it up to par with the door.
Some heavy duty scouring pad (Scotch-Brite type) comes to mind. They even make large round pads for floor scrubbers. You could cut some of that and put in there.
Some air conditioner filter pad available is heavy and green like scrubbing pads; you could try some of that.