Significance Hearing relies on temporally precise and indefatigable transmission of sensory information mediated by inner hair cell (IHC) ribbon synapses. Synaptic ribbons are electron-dense structures that tether numerous vesicles at the active zone (AZ) and drive neural encoding of acoustical signals by neurotransmitter release. The mechanisms of ribbon formation, AZ targeting and subsequent functional maturation in murine IHCs remain elusive. By combining various high-resolution imaging techniques, we analyzed multiple developmental stages and provide a new hypothesis for the assembly of auditory ribbon synapses from the apical cochlear turn. Our findings suggest that fusion of presynaptic ribbons contributes to the morphological AZ development that is essential for the functional maturation of afferent synaptic transmission within the cochlea.