I have a pair of external speakers plugged in, so first I tried unplugging those. For a while, music flowed out of the tinny internal speakers, but next day - silence reigned again.
- tried restarting the computer - no change. Opened System Preferences > Sound, nothing appeared wrong.
- Okay next most simple answer - unplug and replug. When I'd unplugged and replugged the audio speaker jack in the headphone socket a couple of times - Yes! Fixed!
- En route to this happy result, I also checked System Preferences > Sound again and found that the Mute button at the bottom of the window had got ticked. Well, unticking that doubtless helped.