I have a MacBook Pro model 8,1 running El Capitan. I have always had an NEC monitor attached via the mini-Display port to a Display Port on the monitor. It worked fine.................. until.
Recently I attached a JBL 'On Tour' external speaker to the audio-out on the Mac and now I have all sorts of issues.
The monitor often goes blank when I connect it, the monitor often doesn't work (mirroring) when I wake the system up, (there's a message on it saying 'Display Port - no signal), the Mac crashes, the speakers won't work when plugged-in and the volume level shown on the screen displays a 'no entry' sign.
These issues started the day I first used the external speakers. but continue when they aren't attached.
Sometimes it works just fine......... but not often.
Any thoughts?

Power requirements? What do the speakers need, and does that exceed what the MacBook can deliver?

Now why do problems persist with no speakers attached? Maybe a reset of the SMC (System Management Controller)?

The speakers are mains powered, tried the SMC no change.
Thanks for the idea though.

Have you checked JBL's website to see if there are any known issues?

Have looked at the JBL site, looks like the 'On-Tour' speakers are too old to get a mention any longer.

Others, who know more than I about the technical structure of the MacBook Pro audio ports, will be better able to answer the possibility I raise. However, the fact that your issue continues without your speakers connected suggests to me there may have been a short in the initial connection, sending a spurious signal that affected your software.

I checked images of the On-Tour speakers and it is clear that the mini-plug is the tip/ring/sleeve configuration of a standard stereo connection.

However, I believe some Apple audio jacks are tip/ring/ring/sleeve. If that's the case with your machine, there may have been a mismatch.

Anyone?

An observation only but the tip/ring/ring/sleeve jack is designed to be compatible with either a tip/ring/ring/sleeve or tip/ring/sleeve plug so that would be an unlikely source of the problem. There could be a conflict inserting a tip/ring/ring/sleeve plug into a tip/ring/sleeve jack. However having inadvertently done that more than once the only ill effect I have encountered was a very loud and annoying buzz from the speakers.

I suspect plugging in the speakers was only coincidental. I thought Ira's suggestion to reset the SMC would work but since it didn't I only have one suggestion. Back in the day there used to be a sovereign solution of doing a complete shut down including disconnecting all external devices, disconnecting the external power supply, and in the case of laptops removing the battery and letting every thing sit for anywhere from 15 seconds to three minutes before putting everything back in the reverse order in which it was disconnected and then rebooting. This allowed some hardware registers to drain down and sometimes cured otherwise intractable problems.

Since it does not cost anything, it would seem to be worth trying, but since the battery in your MacBook Pro cannot be removed you will have to bypass that step. 😀

Any more info or resolution?

I wonder if there's any possibility that something in Rain's audio out port is stuck in the wrong position?

Maybe plugging/unplugging a few times in quick succession will help.

On the face of the situation, the audio out port on the MacBook Pro 8,1 should not effect the video output — at least in theory but according to Mactracker the "mini display port" is actually a Thunderbolt digital video output which is compatible with minidisplay and it also carries an audio signal as well. So there is a possibility one might effect the other.

Rain (the OP) indicated that sometimes everything works well and sometimes not and while a software glitch is not beyond the realm of possibility that is a symptom more often associated with hardware issues. I have a couple of possible theories...

1. when the JBL On Tour speakers were plugged in the audio in port or more likely its connectors to the logic board were inadvertently damaged. This would be a relatively good thing as it would likely be an easy and inexpensive fix.
2. as happens surprisingly often the problem with the NEC monitor and the audio are coincidental and unrelated to plugging in the JBL speakers (a.k.a. the worst case scenario.)

That leads to a coulee of troubleshooting queries…

1. When the external monitor is DISconnected
   1. does the MacBool Pro display work reliably?
   2. are there any audio issues?
2. Have you tried a different cable between the Thunderbolt port on the MacBook Pro and the NEC monitor?

Considering the assortment of seemingly unrelated issues that Rain is experiencing I wasn't particularly optimistic about my suggestion working, but also considering that they all seem to be interconnected in their unrelatedness...anything's possible.

Edit: Also in an anything's possible vein, I'll add to your suggestions my own that Rain run the OS X El Capitan 10.11.6 Combo Update.

Sorry for my tardy response, I have been away for the past 3 weeks. Thanks for the replies, some useful ideas. What I have now done is switch the cable for the Mac to the screen to the HDMI from the display port, it seems to have fixed it........... so far. (two days), As was said these issues which have many possible causes are really messy, but the forum helps to sort these.
Thanks