If you look to overeat headphones, they all have this tube around the audio device to seclude the ears from external noise.

Could It be a way to include this in the setup of the intercom? To have very soft, but ear-cushions attached to the speaker that direct the sound more to the ears but Also blocks external audio. Ideally reaching the skin and actually providing audio isolation to a certain extend.

I could image that it would be from a soft material on the outside and maybe inflatable to best fit once the helmet is on. Or not inflate it if you do not want to use it.