GREETINGS.
All of last week, I was able to play audio successfully through one of my sound systems (call it System A) by plugging in the 3.5mm audio connector to the to the audio out port on my Pro Macbook 2015 15".
This past Friday, I plug in a different system (call it System B) by unplugging the cable from System A and plugging in the 3.5mm cable audio connector from System B to that audio port.
A and B are totally separate systems. They are one-way audio out to speakers only, and both appear to have the same type of plug. There is no damage to the plug of either system.

System B works fine. I unplug it and shut down the computer Friday.

Here I am two days later ready to listen to audio through System A. There is no audio. And when I unplug the cable, usually a dim red light from inside the port turns on then off after 0.4 seconds just as I unplug it.

But if I plug in System B, it works. (But I sometimes still get a red light showing from inside the audio port if I unplug the plug leading to System B.)
But if I plug in System A, it no longer works.
But if I plug in System B, it works again.
Regardless of which plug, the computer recognizes that audio should be delivered to the jack.
I want System A to work just as it has been all summer.

I tried plugging/unplugging the plug leading to System A many times and in different orientations. That didn't work.
I tried restarting the computer, 5 times, once after each attempt to see if plugging/unplugging the cable would help. That didn't work.

What's going on??

Update:

Mods, feel free to delete this thread. The problem is fixed.

Somehow, by an act of shear inexplicable magic, the 24V DC power cable overnight disconnected itself from the back of the speaker in System A and fell to the floor, so that I did not see that a plug went missing. That 24V plug is a TIGHT fit, and I have no memory of me watching myself moving the speakers over the weekend, so there was initially no reason for me to assume that the speaker connection within System A was the problem.

So I've been fiddling around forcing things into the port on my Macbook to fix itself and wearing it out prematurely for no reason.

[reader50]

Your 3.5mm audio jack is a combo jack. It has contacts to work as an electrical jack for traditional cables / headphones. It also has optical capability, so you can plug in a mini-TOSlink cable for digital audio-out. The red light is the optical system offering data to the cable, in case it's a fiber optic cable. This is how you can hook up a receiver, and get 5.1 or 7.1 surround out.

I'm assuming your cable is a traditional one, with metal contacts on the end? The optical version would have plastic ends, with only the very tip being metal.