We just hung the HT door, the door opening was a bit bigger than the door itself so we are left with aprox 1 to 2 inches of air space between the door jam and the wall opening. Anyone have thoughts on what is the best way to insulate this gap for sound? I've thought about using thin sheets of fiberglass insulation or GreatStuff foam, any other thoughts?

Second question, once I fill this gap with something, I will probably put GG on the casing before I nail it up around the door frame, or will this really do anything?

Just a bit of fiberglass. Uncompressed. And make sure the drywall goes all the way to the jamb to cover all that space.

thanks for the suggestion Ted, unfortunately the drywall doesn't reach all the way to jam since we hung the door after the drywall was hung. I can't think of any way to hang a 1 inch piece of drywall to cover up the small little gap. Any thoughts on what might be a good cover after I put the fiberglass in the gap?

Plywood? Seal it to drywall and jamb with sealant

I hadn't thought of that, I'll give that a go. Thanks Ted!

Great. Ask more questions. Maybe take a couple pics

You could also cut some 2x4 stubs and shove in the gap to give you something to fasten the strip of plywood to. Just don't force them in because it will bow the jamb.

You really should do this right. Take the door back out, and install 1x4's to reduce the rough opening so that it's no more than 1" (total) wider than the door frame. Fill the space between the jam and the R.O. with minimally expanding foam. After installing the door casing, run a thin bead of caulk around the perimeter to seal it completely.

Not so sure we need a solid block at that juncture. If he needs more of a nail base he can install a few nailers as Big described. I might pass on the foam as well. We'd still like big insulated air cavities and not risk coupling too much.

The key is the continuation of the drywall to the jamb then seaaling the remaining small gaps with sealant prior to casing installation. Casing is just cosmetic.