Normal dimmers 'cut out' the first part of the half sine wave; the TRIAC turns on when the voltage reaches a certain voltage, stays on until current drops below some value and it turns off again. So the switching frequency is twice the mains frequency. The usual circuit is like the one described here http://en.wikipedia.org/wiki/Dimmer
Once a TRIAC is on, you cannot turn it off again with some control signal like you can with a transistor. So there not much choice in the control frequency; at most you could switch at a integer fraction of the mains frequency like 30 or 15Hz but the power delivered to the lamp would be irregular and not much of dimming capability left over.
The buzzing sound is mostly generated by the inductor that's part of the dimmer, it's there to supress EMI and required to pass certification. The filament of the lamp can buzz too.
You could use some MOSFETs to PWM at a higher frequency, but that won't be a simple circuit.