Onboard speakers just stopped working?

Argh looking for help!
3 month old C4. Was watching stream just using the (terrible) built in speaker sound out and it just randomly dropped out. No indication of anything wrong. If I connect to BT remote speakers it streams fine.
Doesn't matter what the source is (onboard stream, HDMI, etc), speakers just have no sound.

Any ideas?
Seems like an odd thing to just break randomly.