I’ve just finished watching the first video on building Music Room 2. Paul mentioned that ideally the corners behind the IRS V would be curved and not at a 45 but felt it would be too expensive. Now I’m not trying to spend anyone’s money for them but doing an concave corner really isn’t difficult.
The top and bottom plates for the curved section are made out of plywood or OSB. Whatever radius is required is easily jigsawed. Stand up the studs at whatever spacing is deemed necessary.
To bend the sheet rock to cover the curve place ends of the sheet of Sheetrock on sawhorses, spray both sides of the sheet with water from a sprayer or spray bottle and let sag under its own weight. I have also seen people use a small amount of weight to assist the sagging. The vids give a brief idea.