I was playing a game yesterday and everything was fine.
I paused the game, put the controller down, and left everything as it was.
Today I came back, plugged my simple headphones into the controller, and there was not sound.
I rebooted the box but still no sound.
It turns out my controller has a firmware update ready to install, although there was no indication of that, and after installing sound was back to normal.
You can check and update the controller via the Settings > Devices & Accessories menu (or similar). Find your controller and there will be a tile for updating it.