The orginal question was about french doors that are soundproof. The first answer was to find double pane glass doors. But then the discussion got diverted and these questions were never answered. There are commerial products that address these questions... I give some examples below... and of course there is alway custom if you have lots of $$$.
French doors are usually thought of as doors with glass panels. Some of the discussion was about the company Masonite and "safe & sound" panels... this is not the answer because while Masonite does make french doors they are not double pane, and the safe & sound panels are not see-through. Here are better choices...
ThermaTru has a limited series of glass panel doors (french doors) called "Noise Reduction Door System" which involves specially laminated glass. Grills are added to give the appearance of 10 or 15 panes of glass (in french door terminology, this is called 10- or 15-lights). This product has a STC rating of 36.
Another choice is the Pella Designer series. These are all mostly double pane glass doors with grills mounted outside the glass or between the glass, to give the french door look. Options include internal blinds or internal screens that fit between the two glass panels. They even have an option for TRIPLE pane glass doors. The Pella Designer glass doors have a STC rating of 34-35. The other Pella series that include french doors (Architect and ProLine) have lower STC ratings.
Btw, the STC rating stands for "Sound Transmission Class" which is a logarithmic scale used by acoustic professionals to compare things like soundproofing ability of different materials. For example, loud speech can be understood through a wall rated at STC 30 but should not be audible through an STC 60 wall. A typical wood framed wall with 2x4 studs, two 5/8" drywall-sheetrock wall panels (one for each side of the wall) and batt insulation in between has an STC rating of 34-39. There are various construction techniques to get the rating up. There are now specialized drywall materials such as QuietRock which bring the STC rating up to 49-52 without special construction techniques if you simply substitute one conventional drywall-sheetrock panel for a QuietRock panel... even higher STC if you substitute both.
So what about glass? For comparison, the normal STC range for a single pane of glass is 26-28. The STC for double pane glass ranges from 26-33. Again, this is a log scale so small differences in the rating number mean a big difference in the sound transmitted. Double pane glass french doors or engineered glass french doors like Pella Designer and ThermaTru with STC ranges of 34-36 are pretty good and comparable to a conventional wall in your house... compared to doors with single pane glass which are going to suck.