I just learned that the difference in dB is 20 times the log of the voltage ratio of two sounds. But, it just occurred to me, what is the unit of the amplitude on the y-axis in a waveform? Is that "voltage", or "watt"?
None of the above. The waveform display program shows the raw sampled numbers (not anything in decibels), but there can also be an intensity curve, in dB, superimposed on the waveform display. In Praat, the string of numbers that is the waveform is typically scaled to a real between -1 and 1, but underlyingly (in the input file) it is a 16 (or so) bit signed integer. The numbers are produced by the A-2-D converter, which maps varying input voltage to integers, and the microphone converts varying sound pressure into varying voltage. Thus, a unit of sound pressure level, defined here.