Thanks for the replies everyone.
mindwaves: Skype offers as sound options only the choice of input/output device.
analogika: the headset is a USB one (Logitech). Nevertheless, it seems that the input signal is not so strong, since people complain they can not hear me loud enough, even when the input volume is set at maximum in System Preferences.
Well, comunication is still possible, just not optimal. And no, I have not the intention to blow their ears, just to make them hear me like through the normal phone line. I think being able to up somewhat the input volume would resolve the issue. I remember sometime ago there was a utility that let you increase the movement speed of the mouse cursor beyond the limit set by Apple. I thought there is maybe something similar for the input sound volume.