What's a good sound proofing material to fill an old aperture?
Rigid Foam Insulation Cut to fit the gap, use as many layers as needed, cover with the grille. If you determine that you will never need the hole again, take off the grille and finish the wall with a drywall patch or (if the wall is made of some other material), paint the foam directly (many types can be painted) or (lazy way out) hang a picture over it.