How to soundproof a door
The best solution to access is simply to buy a sound proofed door.
This will be expensive, but it will be worth it. It will probably have magnetic seals around the top and sides, and a compression seal at the bottom.
It will also be very heavy, meaning that the wall it fits into will have to be strong enough to support it.
A reasonable alternative is a heavy fire door, with the jamb fitted with rubber compression seals and extended all the way around the door, including the bottom.
To gain better insulation than a single door can achieve, a 'sound lobby' is sometimes constructed so there are two doors between one side of the wall and the other.