How do I get the sound to work again?

I've got a Lumia 950, which I have to keep going until I can get another phone. Today it presented me with a new problem. It's acting as though I have it muted, but it isn't muted. The vibration indicator is on, but if if I go to the Control/Action Panel, it clearly shows that it isn't muted, but there's no sound. By that I mean that if someone calls me, it doesn't sound the ringer. If someone texts me, it doesn't produce the sound of a text message arriving. It does vibrate, but that's all. I've shut down my phone and turned it back on again, twice, to no effect.

I'm hoping I've done something accidentally, to turn off the sound. If that is the case, I'd like to know what I have to do, in order to get the sound for phone calls and text messages working again.