I finally asked one of my attendings what the hell he meant by a sonometer one day, and I think I embarassed him. His answer was basically "it's like a centimeter.. but a sonometer.. just..just use centimeter, these (pointing at a ruler)". I honestly don't think he knew for sure if there was a difference, despite understanding how to say both. As it turns out, a sonometer does exist, but as an instrument which (basically) measures vibrations of a cord. To simply pronounce "centimeter" differently one should still include all the syllables, otherwise you risk saying a different word altogether.