That's indeed a great tool to have @olujay. It's great that you've grown to nurture your childhood curiosity. Your multimeter looks fancy it has an auto power off feature. It is easy to forget to switch them off after use.
The mixer has some internal issues with its earthing
Were the earthing issues specifically from the mixer or maybe they resulted from a ground loop when connecting a grounded microphone?
Did you end up with a setup of an ungrounded microphone that connects to a grounded mixer for the microphone to work properly?
Yes, it has an auto power-off feature, which helps to conserve the battery when I forget to turn it off. Pretty cool.
Actually, the mixer and its earthing was a guess. I figured the problem should be around there, and I sure didn't want to go into it. I am not an expert in that area of electronics. Somehow, the thing just works better without the grounding.
I see you do have your background in electrical and electronics. Did you study that or something?