iPhone 4S - is there a way to control sidetone?

Just "upgraded" from my old 3GS to the 4S ... and immediately noticed that there is now sidetone. I use in-ear isolating earbuds (Klipsch) my old iPhone had little or no sidetone ( Sidetone is where you can hear yourself talking in the mic. back in your own ears ) Now with the new 4S it's there... and it's really annoying.

I've spent some time with tech. / I even swapped my (essentially brand new) phone for another one thinking it was a mfg. problem. * the problem disappears if I use my headset with my friends iPhone 4 ... so it's not hardware.

All to no avail. ( although kudos to the guys at the Genius bar and phone support for humoring me about it ... and they agreed : it is very strong with in-ear sound deadening earbuds. - it in fact renders them worthless. )

I'm assuming this is a software controllable setting - does anyone know how to access it?