I have a Gentex HGU-55 bonedome (for Yak flying) with a set of
THESE soldered in parallel with the earpieces. You put the ear pieces in, the bonedome on, then plug the earpieces into the socket on the side of the helmet. The existing speakers still work if you don't want to use the earpieces (handy with a rear passenger if they don't have the same system and need the radio turned up). It allows me to fly with the radio volume turned down to minimum and makes a huge difference.