NOTE: I didn't think to do this until after I started writing this guide so I didn't do this on my keyboard. Despite that, my volume knob is usable again. It will still get a little confused about which direction it's rotating once in a while but it's negligible and not causing problems. I can't help but wonder if it would be 100% if I had.

Using a Q-Tip and some rubbing alcohol, gently clean out the encoder traces (the area in between the two red circles in the image). The goal is to remove the existing dielectric grease.

Once clean, apply new dielectric grease to the encoder traces. Apply about the same amount as what you removed during the cleaning process. Adding a little extra shouldn't be a problem as long as you don't go overboard with it.

CAUTION: Be extremely careful not to dislodge the small spring on the outer edge.