SuperFinn asked me the other day about sound proofing a 2cv, and I had done a bit of this a while ago following on from my friend Julian soundproofing his dyane.
Then thought that this may be of interest to others so I'd post here.
I got hold of some fireproof sound deadening material from an auto factors place, it was kind of like a thick carpet / matting material about 1cm thick. Pretty cheap and you can still get it in general motor stores.
Julian told me that soundproofing material should be attached to panels, not necessarily to a whole panel, but even attaching partly to a panel will stop it resonating and therefore reduce the noise.
So if you've ever taken a door card off a 2cv you'll see there are three vertical rectangular shapes first step is to fill these in with cut out shapes of your sound deadening stuff, spray adhesive works well. You can do the same with the back doors and using three triangle shapes for the bootlid.
You can add the stuff under the front floor mat but I'd only do this if I was absolutely sure there are going to be no rainwater leaks as soundproofing material wet would be a fantastic rust incubator. Under the rear floor mat and under seats should be OK though.
The bulkhead is pretty well covered with soundproofing already, as is the inside of the bonnet (unless you are SuperFinn!) as those are the best areas to reduce noise.
The other areas I soundproofed were the inner front wings and triangles as you can see. The cut out shapes are really just to reduce the resonance in these panels rather than physically block the sound.
Once the whole lot is installed the difference can be quite marked, the next steps would be rear box section and inner rear wings, and depending on inclincation, replacing whatever is left of the headlining with soundproofing stuff too if you can find some which looks nice.
Or just ignore all of this, rip out the existing stuff under the bonnet and disconnect the crossbox for More Noise
Wind and tyres 
If i want silent car, i would not drive 2cv...