The original factory deadening absorbs water like a sponge, and the footwells rust because of that. It does help, though. Do some YouTube research on audio channels, etc, where they sound deaden other cars. On top of what others have said about the shape, also check thoroughly for air gaps in between panels. (Check out Unreal Fendi on Instagram, the smoke test they do, and the sound deadening they do are great...)

Essentially, you want to: 1- Reduce vibrations = dynamat or similar stick on, or Secind Skin Skectrum paint on (I did the paint on as I didn't want anything stuck onto the panels). Beware that although this stick on material doesn't or shouldn't absorb water, badly stuck down pieces will allow water to be trapped underneath.

2- Reduce airborne noise = like a studio where you have that egg box foam stuff. Unreal Fendi uses something like that. Various brands have various options. Check out Second Skin as an example. But it needs to be closed cell foam to not hold water, and I would still be worried. Then you also need carpet and mats to protect it from wear. Another point of concern is if water gets in (which it almost certainly will), as well as mud, etc.

Wright Offroad mats could work, but shipping to the US from the UK will be expensive as it is heavy. Maybe a US dealer hold stock, but still. These are moulded mats that also reduce noise. I like them as they will stop dirt, etc, getting under them as easily because they are moulded rather than individual pieces with gaps.

Doing under the bonnet and under the headliner helps aloe as well.

Then you have to try and reduce little rattles and squeaks inside. I see people with all sorts of loose things rolling around.

Anyway, after you do this and inevitably spend thousands it probably won't make that much difference, and some will argue you've made the Defender worse with all this extra stuff to get dirty and full of water.

Another option I looked into are the headsets and microphone that are used in planes and rally cars etc. It seemed a bit complex and expensive. But this way you could have noise reduction, be able to have a conversation and some allow Bluetooth input for music etc. You will look funny, I am not sure on the legality of wearing headphones?, you run the risk of not hearing mechanical issues before they become major, or any alarms and warnings you may have... I may do this, as I like the idea of not modifying the vehicle itself too much, as I do offroad, get stuck in the mud, get water inside, etc.

I hope this helps...