I'm sure you'll get lots of suggestions for "off the shelf" things but the only things I've found are either helmet speakers which are OK if your helmet is not noisy or the Green Leopard monitors - not cheap but they double up as an custom molded ear plug so they cut out unwanted noise and pump your tunes / gps / pillion straight into your ear. This means you can have the sounds you want to hear at a lower volume and you don't hear the sounds you don't want to hear. For long journeys these can't be beaten IMHO. Beware of cheaper imitations with thin cable as they will fall apart.
Probably not the answer you were looking for but food for thought...
That's wierd because i had a set that did that, took them back to the sony shop kicked up a fuss said i paid in case and could not find reciept and they swapped them for a new set.
I had mine for 18 months before one died though...!