Significance We hear sounds varying in intensity over six orders of magnitude using spiral ganglion neurons (SGNs), each of which changes its firing rates over only a fraction of this range. Somehow, the SGNs with different dynamic ranges collectively encode the full range of sound levels represented in the receptor potential of the inner hair cell (IHC) in the mammalian cochlea. Our study, combining subcellular imaging, mouse genetics, and auditory systems physiology, offers a unifying synaptic hypothesis for wide dynamic range sound encoding in the spiral ganglion. We propose that IHCs, from one receptor potential but via presynaptic active zones that vary in the voltage dependence of Ca ²⁺ influx, generate complementary codes on sound pressure level in functionally distinct SGNs.