I think the interior soundproofing might vary by brand. When I swapped speakers in a Rugged Radio headset a couple of years back, I don't recall any major stuffing in the cups. Looking at pictures, I see a sort of doughnut-shaped foam ring that went behind the speaker. Don't recall anything else. You can see the ring on the top left of this picture:
You can see the ring around the speaker in the cup.
You could try stuffing some sound-deadening material inside the cups. It's probably the easiest way to go, as long as you can find a place that'll sell small quantities. I once used little blocks of dense temperfoam to try reduce the noise level. Seems to help a bit, but never did a scientific measurement.
The second approach is to remove the speakers ("receivers") and install a set of ear buds, instead. Check out this picture:
If you look carefully at the bottom center, you'll see the ear buds that dangle out of the original ear cups. These dangle down about ~5 inches below the helmet...set the helmet lightly on your head, insert the buds, and pull the helmet down gently over them.
Now, civilian ear buds don't have the right impedance, so you need an adapter. This is just a simple, tiny audio output transformer with a 300-600 ohm primary and an 8-ohm secondary. Used to be able to buy them at Radio Shack, but online places like Digi-Key and Mouser electronics sell them for ~$4 or so. Here's the diagram to make it work:
The part number that I use is right on the drawing....XICON 42TL004.
These transformers are small, and easily built-in to tiny boxes. You can build a separate adaptor as show above, or just build it into the headset itself. If you can't find one, drop me a message and I'll stuff a couple of spares in an envelope and send them to you. I made a lot of runs to Radio Shack before it closed. :-)
The photo that shows the ear buds has the transformer located between the lining of the helmet and the leather, right at the nape of the neck. This is also the helmet that I stuffed the dense temperfoam into the cups to add to the passive attenuation.
The neatest thing about this approach? They make ANL ear buds! But I use
Plugfones, which are foam ear plugs with speakers inside. They have 24 dB of passive attenuation (same as a passive headset) and cost $25 a set.
Ron Wanttaja