Unfortunately, I have never required an app to play YouTube videos which were stored on my phone because it never made sense to me to waste the storage space.
I’ve always found that the ones which played them from the YouTube site to work just fine for me but that’s because most of the time I’m using my phone with wifi.
I guess if someone wanted to avoid data charges then the priorities would be different but for me that’s not usually the case.
BTW: The developer for the Playtube app to which I referred is Sepia Software. So, if you put that into Search along with the app name, you should have no trouble finding it.
And it can play in the background when using other apps so, logically speaking, itshould also allow turning off the vid and listening to audio only.
However, I really have no idea if it plays videos stored on your phone. You can just download the free version and give it a try.