The US Army, I seem to recall, has available earphones which go into the ear canal andsubstantially reduce the volume of outside noise reaching the ear, and also the volume of sound required on intercom/radio to hear clearly - see
http://www.cep-usa.com for more details. As an example, I bought some Shure headphones of a similar design, halved the volume on my MP3 player even on the tube while still able to hear well enough to speak to other people - the results in an aviation environment I would imagine are similar.
Advantages - no batteries, passive. Disadvantages - require integration with headset / helmet, and need careful fitting to ensure comfort for long periods.