Vocal frequence almost gone

Hi, I have these once wonderful headphones, but it seems they now won’t work as before. They seemingly charge, with the light showing red, but the power button really doesn’t evoke them, except very seldom. Sometime also randomly, on its own and then they won’t turn off.

As for the sound quality, the whole middle register, especially vocals, seem to almost vanish into the background. This is regular, even when playing cable only.

I’ve disassembled them according to the given instructions without finding any obvious weak connections. A little rust on the head of two of the long screws was all I could spot.

Any hints? Are they worth repairing, or should I consider recycling them?

-Glenn