There is a solution in the remote Apple - iPod - In-Ear Headphones with Remote. I just trying to figure how i would do it...

If the iPod headphone resistance is say 100 ohms, it is incremented by 1 ohm by means of a tiny push switch on the headphone or in the cable, it will not disturb the iPod sound. This 1 ohm increase in resistance can be sensed by a DC 10uA constant current which will not be affected by the AC music circuit. Now by a click and double click we can signal a volume or next song. With two switches 101 ohm and 102 ohm you can get more control. There is no special connector for the headphone and it will work with older iPods as well. And as it uses impedance Signalling, there is no new noise in music.

An option for Ambient Acoustic Noise cancellation in iPod would make the music sound better. The microphone can be in the iPod and only the larger high pitched noises can be compensated. The same Mic can be used to auto adjust volume with ambient noise.

A certain volume setting during the day is not valid in the night as ambient noise is less and ears become more sensitive as sun sets.

The addition of a microphone in iPod will help using Google Talk for the iPhone for PC-to-PC voice calls.

Apple - iPod - In-Ear Headphones