How to fix headphones with split-jack 3.5

Soo my jack in my headphones stopped working, so I cut the insulation off and looked at the wires. My headphones have 1 cabel, that splits into 2 (mic and sound jack) and then you can add a reduction so it connects once again into one.

I am sure the problem is in the 2-jack section, because before it stopped working it sometimes got really quiet so I could barely hear anything, and I could fix that by rotating the sound jack so it can get better connection.