French Doors

I suggest 5 inch thick steel door with three custom locks and keys. Anything less - the kids and visitors will find their way there.

You don't sound like you're into construction, so definitely hire a good contractor. He will re-frame your opening to the correct door size, and then fill in the area with drywall. Depending on your wall finishes, he will match the existing look. Honestly, this could end up being a big mess unless you have the expertise to do it right. I'm a retired carpenter, so I know whereof I speak.

Just my opinion here, don't use french doors. Have a solid door to your listening room.

I second the response from Sid42. Ask your neighbors for suggestions for a good local "handyman" company they use, or check Angie's list. Any decent home improvement/repair contractor can frame in a door, mount drywall, and hang the door with his or her eyes closed. Like Sid42 , I know whereof I speak - I'm a complete klutz, but I have a great home improvement guy that everybody in my neighborhood uses. I may not be handy, but I can speed dial with the best of them. ;)

Don't use french doors and use a standard size door. That way you can treat that wall for a better sound.