There are two options available depending on your platform. One is closed captioning, and the other is a rolling transcript. I don’t have time right now to post detailed instructions on how to access them, but you can search the support community for this. Others can also contribute here. Later, when I have more time, if no one else has responded to this, I’ll post more details.

Be prepared, however, to cringe at “all right” being incorrectly spelling as “alright”. (It hurts just to type this.)

Someone else posted elsewhere the following link, which is for playing videos on the website, not app versions.

https://khanacademy.zendesk.com/hc/en-us/articles/204794840-What-features-does-the-video-player-have-

Here is something I wrote specifically for the iPad app a while back. This may work for the other mobile platforms, but I don't know for sure.

When you are playing a video in the iPad app, tap on the screen to show the time elapsed bar along the bottom of the video screen, as well as some icons in the bottom right corner. The icon just to the right of the time elapsed bar is shaped like one of those balloons you see in comics that contain the spoken words of characters. This is a toggle on and off switch for same language subtitles/captions. To turn off the subtitles/captions, click on this balloon-like icon again.