It is a common problem we have it with our Luton Minor and the easiest fix we have found which reduces it, is to use earbud headphone under the normal earphones and use a 3.5mm to 1/4" adaptor! The volume needs to be turned down as the earbuds are very good (LOUD)! There are a few companies in the States that will sell you purpose made CEPs with sockets etc to modify your helmet but being true tight Scots we have been experimenting with earbuds!!! Sony ones for mobile phones with mp3 player are so far proving to be best.
Make sure the type you acquire dont have solid stems on the wire exiting the body this becomes uncomfortable although you can break them off!
Earbuds are the ones that come with the removable silicon adaptor that allow you to get a snug fit in the ear canal.