My Android box is playing music

I don't know what I did, but while I was trying to update a couple of apps I hit something and now I've got music playing on my Android box. It's one song and it seems to be on Loop.

I don't know what it was that I turned on but I don't know how to go about turning it off.

I thought maybe I accidentally clicked on an YouTube video and that it didn't turn off when I exited the app. But that isn't it. Every video I click on in YouTube I still have this music playing in the background.

On the home screen there is a volume icon in the upper left. Like an upside-down megaphone. I've clicked on that but that doesn't do anything at all.

Any idea what I turned on and how to go about turning it off?