Sound Settings: says speakers unavailable but headphones plugged in

[m654321]

Have you upped all the levels in Alsamixer? eg. The Master level for starters is at 1 out of 100. Make sure nothing is MM (muted)

Thanks Jerry.
Yep, I have upped all sound levels, but still only sound on headphones. You'll see on the screenshot that some of the alsamixer sound level columns remain on muted (MM), notably speaker & mic (I seem unable to unmute them, unless I'm doing something incorrectly): https://imgur.com/a/IMDQ8u4

I think firenice03's idea that it may be a hardware problem, such as a loss of cable connection with speakers, is worth investigating. However, opening up the laptop will be the last thing I do (only if all else fails) as the manual shows access to the speakers is a bit complicated.