Rocket Validator looks for accesibility violations on your pages using the axe-core accessibility engine, hosted on our own servers and integrated into our web crawler. You don't need to buy an additional license, everything's already included in your Pro account at Rocket Validator.
A11Y check: audio elements must have a captions track
Accessibility rule: The track element must be present for each HTML5 audio element, with kind='captions' set as a property. Captions are text—synchronized with the audio file—of the dialog, narration, and any important auditory information, for the benefit of deaf users.