Image on left is from 4 days ago, but the pimple was slowly forming over around 2 weeks.
The goop was sticky, not oily. Earphones are Panasonic RP-TCM130.
I was not able to find an explanation.
Something to increase cable lifespan, lubrication, rubber disintegrating, sweat and earwax that somehow got into the cable, dielectric grease, SCP-1407, no clear answer.
At first I thought the wires just somehow twisted. Nope.
Sounds optimistic. Checking a few USB to 3.5mm dongles, they seem to be around 25mW max at 32Ohms. Not sure how much that would change with short circuit, but I guess not much more.