Sound encoding at the synapse between inner hair cells and spiral ganglion neurons in the mammalian cochlea operates with submillisecond temporal precision, drives neural spiking at hundreds per second over hours and covers sound pressures that span six orders of magnitude. When hair cells transduce a sound driven mechanical stimulus into an electrical signal, voltage-gated CaV1.3 L-type Ca2+ channels open and the Ca2+ influx triggers exocytosis of glutamate filled vesicles at their ribbon synapses. Each of the 5-20 active zones of an inner hair cell drives spiking in one spiral ganglion neuron in the absence and presence of acoustic stimulation. Using functional and morphological fluorescence imaging we found that the complement and voltage-dependence of Ca2+ channels as well as the sizes of active zone and postsynaptic glutamate receptor cluster vary among the synapses of a given IHC. Synaptic heterogeneity emerges during postnatal presynaptic maturation, seems to be governed by topographic rules and may involve the selective presence of synaptic scaffolds. In summary, hair cell synapses differ from one each other in structure and function thereby covering the broad range of sound pressures with the limited dynamic range of encoding at each individual synapse.