[Paul W]

These systems - (phase inverting reproduction/cancellation) do work incredibly well on headphones - I have tried them in the studio (They are ideally suited for aircraft uses). The headphone system works by having a microphone on the outside of the headphone shell that picks up what you would have heard and replays it through the same side of the headphones with the phase reversed - cancelling it out (taking into account that you will only be hearing what is bleeding through to your covered ears - that seal very well). You can also listen to music on top of this as they are designed to be used as normal headphones. If you don't wan't to wear headphones then the size of the speakers you would need to come anywhere close to a screaming V8 would put you off and still could never be as effective.
I have tried wearing headphones in cars and didn't like it - o.k. if you've got a helmet on though.
What would work better imho is to deaden the firewall / floor as much as possible - lining the bulkhead cockpit side with thin mdf board would actually be just as good if not better than any foam you can buy especially combined with a double glazed window.
Obviously theres a weight penalty with this but it depends how quiet you wan't it - personally I think alluminium - sealed with foam gaskets (Or draught excluder) and carpet with some automotive type deadening like the stuff used in speaker boxes should be more than good enough.
I have made an ally engine cover for between the seats after riding in a car with a fibreglass one (and carpet). Also considered it a bit safer too.
Also bear in mind that it's only ever going to be as good as the weakest insulated part - changing to a glass double glazed unit is pointless without improving the immediate area around it.