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?


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honestly, and you'll find people who are eager to
fill your head with information"
--Walt Disney