The absence of the multi-C2 domain protein otoferlin causes profound deafness probably due to failure of Ca²⁺-triggered neurotransmitter release from inner hair cells (IHCs). We now studied deafness in pachanga mice that carry a missense mutation in the Otof gene (Otof^Pga/Pga). This mutation leads to a 70% reduction of otoferlin protein in IHCs, quantified by immunohistochemistry. Membrane capacitance measurements in mutant IHCs revealed unimpaired exocytosis of the readily releasable pool of vesicles (RRP) when sufficient time for RRP recovery is provided. However, the sustained transmitter release and re-supply of vesicles to the RRP were strongly diminished. The presence of normal multivesicular synaptic transmitter release was confirmed by recordings from postsynaptic afferent boutons of Otof^Pga/Pga mice. In vivo, sound-evoked single neuron activity in the region of the cochlear nucleus was very low and observed only at very high sound pressure levels. Onset coding and rates of the sound-evoked spiking both improved after longer pauses (providing a longer period for recovery). Electron microscopy of IHC synapses of Otof^Pga/Pga mice revealed a normal number of docked vesicles and immunohistochemistry showed only a minor reduction in the number of ribbon-containing synapses. To conclude, in this work we identify a specific function of otoferlin in the early step of exocytosis at the hair cell active zones and demonstrate the significance of efficient vesicle replenishment at the hair cell synapse for hearing.