Do you get a lot of “snap, crackle, and pop” with your audio connections? Do you hear disgusting noises when you touch or rotate an audio plug that is connected to your mixer? If so, you probably have oxidized connectors!
Oxygen reacts with metal and causes oxidation, which is casually described as a layer of crud that acts as an insulator and prevents a connector from making good electrical contact with its mating connector. The problem is really annoying with audio connectors because you can hear the discontinuities that the crud creates.
All sorts of techniques have been used over the years to eliminate oxidation, including removal by sand paper and pencil erasers. It’s not a good idea to use abrasives with metal connectors because you’ll remove some of the plating, particularly gold plating. Most people have a can of cheap spray contact cleaner, which probably doesn’t work well or last very long.