Sound proofing my french doors

Your biggest issue will be how to seal the doors. You will need a threshold and sealed astragal and a good seal around the entire perimeter of the door.
I am in the same boat, and made doors out of 1 3/4" firedoor slabs with a sheet of 3/4 MDF green glued and screwed to the interior face. If you are going to spend the time, floor space, and money to sound proof the walls I wouldn't go any less than a 20 minute solid core wood door. I made my own frame with a integrated toe-stubber threshold. I'm cutting the face off the old hollowcore doors and adhering it to the exterior door faces so they match the other doors in the house from outside the theater..

I also remember seeing threads where people have filled hollow core doors with sand or concrete.

If you're going all out on soundproofing (decoupling, double drywall, Green Glue, etc.) don't use french doors. It's as simple as that.

The bigger entrance isn't worth the reduction in isolation performance after you've put so much time and money into it.

I doubt there are any hollow core doors that start life with a strong enough perimeter frame to stand up to normal use with the added weight and would not pursue that strategy. I've actually had some hollow core doors come apart in my life and after seeing what is on the inside it would not support the added weight. In a recent project I got a 1 3/4 inch thick solid Particle core door with wood veneer finish (Think heavy office door), mounted in a jamb with 5 commercial hinges. We also added two layers of 1/2 MDF with Green Glue and the door had a 2x4 acoustical panel on the theater side. Sucker was heavy and it was quiet. We did use Ted's perimeter seals. See my Curve Frenzy project.

If you want a sand filled door, build your own starting with a frame that would stand up to the weight. Concrete is a good sound conductor and would add mass but would not damp the sound like sand.
Edited by BIGmouthinDC - 3/4/13 at 5:16pm

Double doors, but not french doors, as in the kind with windows? I used a solid hemlock interior door on my last build - it had no where near the mass of the particle core firedoor and was very thin around the panels (~1/2"). There are a lot of build threads where people have used solid wood exterior doors, and I don't recall reading any complaints.
I am going to split the old hollow door with a circular saw, straightedge guide and jig to make the cut straight and safe. Not the kind of cut I have the skill to free hand. Then screw and glue the face of the old doors to the new. It's a work in progress so who knows if it will turn out. I know it's a hack job, but I would like the doors to match all the other doors in my house.

Good advise. That last reply was meant to be somewhat tongue in cheek, but it never comes across right over the internet. I can imagine a standard hollow core door holding up until the door is hung and closed the first time, then blowing apart at the bottom and spilling 100lbs of sand over the new $45/yd patterned carpet. At my last house one of the doors had a hole in it from being hit by the end of a broom.
I had to google search anyway just to make sure I wasn't imagining things: http://www.avsforum.com/t/1235296/how-to-make-a-2-panel-door-into-a-sound-proof-theater-door

There are double doors that create an air lock between them. One opens out one opens in. The combination has a higher sound control than a single.

Yes I was able to add extra long screws to the hinges that extended into the framing. The jam was made of a paint grade finger joined wood, don't recall the species. It was what the custom millwork shop normally used, probably pine.

I've never paid over $180 for a custom ordered door in a custom width jamb.

You need to figure out where the builders buy their doors in your area. It isn't Home Depot.

I'm pretty sure EVERYTHING in my house came from HD. So far, I've been able to match up anything I've needed to.

There are only so many companies that make the actual door slabs, There are many more companies that buy those slabs, mount it in frame, drill the lockset holes and call it a pre-hung door under their brand name. When I was looking for matching doors, Every one had Masonite safe and sound doors to match.