We use standard units in the acoustic solver, which means we're solving the wave equation in pressure which has units of Pa.
Note though that the acoustic equation is Linear (careful, this doesn't apply to the nonlinear solver), which means that you can arbitrarily scale the input signal by a scalar and the output (in pressure) will be exactly the same and scaled by that same constant. (You need to be careful when you consider energy related quantitites which are something something pressure squared)
Essentially, find the scaling factor between pressure and voltage (assuming linearity), then run your simulation with an arbitritrary amplitude and then scale the pressure output.