I stole this from
http://ask.metafilter.com/20346/How-...s-from-buzzing
Your dimmers may be running at close to their rated capacity. (To find this out, try removing some of the bulbs and see if the dimmer gets any quieter.) You need a beefier dimmer, probably, or else split up the lights into two or three banks with separate switches.
Perhaps the dimmers are the (common) triac-based ones? Are you sure the buzzing is coming from the dimmer, or is it coming from the lights? If it's the lights, then it's the current waveform generated by a triac dimmer. You should be able to find autotransformer-based dimmer units, which would be silent.