Another example is reflection of sound off a rotating moving surface, say bouncing sound waves off of fan blades, that gets a ring mod effect all right.
I'd say before reading schematics, maybe some reading up on Harald Bode, the father of the electronic ring modulator would be in order. He was a contemporary of Robert Moog and developed some instruments of his own. When I worked at the SUNY Albany electronic music lab, there was a Bode ring modulator atop the rack of Moog modules. It took a deft touch to use it well, as it could turn audio into annoying ear racket very easily. Those rare individuals who could use it well, got some amazing sounds in their compositions.
Perhaps your prof. is referring to the sum & difference frequencies that result from mixing audio signals, I guess that's a rough form of modulation, but hardly compares to the subtle through startling effects that can be accomplished with an electronic ring modulator.