Adaptive myelination is the emerging concept of tuning axonal conduction velocity to the activity within specific neural circuits over time. Sound processing circuits process signals with microsecond precision: a time scale that is amenable to adjustment in length and thickness of myelin. Sound-evoked responses during postnatal development increase the activity of auditory axons and enhance myelin thickness, while sensory deprivation prevents such radial growth during development. Deprivation occurring during adulthood reduces myelin thickness. However, it is unclear if sensory stimulation adjusts myelination in a global fashion (whole fiber bundles) or if such adaptation occurs at the level of individual fibers.Using monaural temporary deprivation in mice provided an internal control for a) differentially tracing structural changes in active and deprived fibers and b) for monitoring neural activity in response to acoustic stimulation of the control and the deprived ear within the same animal.The data show that sound-evoked activity increased the number of myelin layers around individual active axons, even when located in mixed bundles of active and deprived fibers. Thicker myelination correlated with faster axonal conduction velocity and caused shorter auditory brainstem response (ABR) wave VI-I delays, providing a physiologically-relevant readout.​The present work shows that activity-dependent myelination occurs at the level of individual axons and that activity in one input channel, i.e. one ear, contributes to proper myelination of activated axons. However, activity in one input channel cannot compensate for activity deficits in another, thereby causing imbalanced myelination and mismatched auditory temporal processing.