Abstract: This study aimed to provide unambiguous evidence for musical training’s impact on auditory brainstem function. The answer to this question bears great significance for sensory learning; if musical training has the power to fine-tune subcortical structures to better process sound, this would attest to the power of cognitive experience to shape basic sensory function. By demonstrating timbre-specific subcortical tuning in musicians, we reveal that the human auditory brainstem is exquisitely more refined than previously assumed. This plasticity is likely driven by cortical-brainstem reciprocity that is strengthened by musical practice. Furthermore, we reveal, for the first time, an objective neural marker for sensory fine-tuning in musicians that unambiguously relates to specific instrumental training backgrounds.