iMac output sound stuck on mute

I recently updated my iMac to Mojave 10.14.1

Since then my output sound is stuck on mute in the Sound pane of System Preferences. The mute button is greyed out, so no option to untick.

I've tried restarting the machine whilst holding down Command + Option + P + R.

I've tried unplugging the power cable, waiting 15 seconds and then plugging back in and restarting.

No luck with anything else I've tried.