That’s a Spotify Leak /s

This reminds me of the old Apple Macbook chargers where the cable was permanently attached to the charger itself. These cables became brittle and discolored from white to yelloy (they looked like they have been in a heavy smokers’ household when exposed to UV light, which is always). When I tried to fix the brittle cables with tape, it got worse and probably due to a chemical reaction between the material of the cable and the sticky glue of the tape they completely disintegrated.

Your headphones are beyond repair. It looks like they have been exposed to some chemicals. Did you at one point in time clean them with perhaps an aggressive cleaner (toilet/ household cleaner)? It could be possible that some cleaner got inside the cable through a micro-crack in the cables’ insulation and reacted with the copper inside.

You should replace them. The ooze is probably not healthy.