The auditory function develops and matures after birth in many mammalian species. After hearing onset, environmental sounds exert profound and long-term effects on auditory functions. However, the effects of the acoustic environment on the functional development of the peripheral auditory system, especially the cochlear sensory hair cells, are still unclear. In the present study, we exposed mouse pups to frequency-enriched acoustic environments in postnatal days 0–14. The results indicated that the acoustic environment significantly decreased the threshold of the auditory brainstem response in a frequency-specific manner. Compared with controls, no difference was found in the number and alignment of inner and outer hair cells or in the length of hair bundles after acoustic overstimulation. The expression and function of prestin, the motor protein of outer hair cells (OHCs), were specifically increased in OHCs activated by acoustic stimulation at postnatal days 7–11. We analyzed the postnatal maturation of ribbon synapses in the hair cell areas. After acoustic stimulation, the number of ribbon synapses was closer to the mature stage than to the controls. Taken together, these data indicate that early acoustic exposure could promote the functional maturation of cochlear hair cells and the development of hearing.