If you're looking to add closed captioning to your uploaded videos, and those videos only have music playing in them, Enable Default Video Captions will caption those videos automatically.
In your Mediavine Dashboard go to Settings > Video.
There you'll find the Basic Settings for Video. Look for the Enable Default Captions at the bottom, and click Enable then scroll down to Save your settings at the bottom right.
This setting will apply a caption to any video that you don't have specific caption files added to in the Mediavine dashboard, and should ONLY be used for instrumental music-only videos. The default caption will be: ♪