I had to use my old Mini today (Mountain Lion) and put the speaker jack into it, from the new Mini (El Cap), which worked perfectly. The new Mini however now refuses to see the external speakers and System Prefs only offers me the internal ones. Please tell me I haven't destroyed something!
Solved, but I can't work out why. Multiple restarts did nothing then I pulled the jack out of one socket and stuck it in the other and bingo. I'd done this before to no avail.
In some of the Macs the 3mm sockets have a switch sensor in them, occasionally the switch gets jammed into the off position and needs to be reminded of it's correct role in life by being exercised a little bit by a few insertions and removals.
Ooooo, you are a one, Mick! Aargh NO! Just looked it up and you're right, but how is a simple man to know what all those funny little icons mean?
And thanks, Derek, I very much suspect that you too are right, because its been in that socket before and refused to work.
Eric Jervis 