Today I tried to play a video but my media volume was on zero.
I used the volume button on my phone but it did not increase the media volume. I then went to the settings --> sound --> media but I could not change the media volume from zero.
The reason i could not change the media volume was because media volume slider was grayed out... So I could touch or change it in any way.
It's a very unintuitive problem and many others seem to have it if you Google.