Soldering back wires to an earbud's PCB

Hello,
That's my first thread and post in this forum.
I used to be a happy user of an Xears CP-100 pair for 7 years now, until the metallic housing opened and the wires got severed from the circuit board.
For some reason I decided to clean the inside of the housing before soldering. Thus, I scratched this glue-like yellow stuff that was covering two of the contacts.
When I tried to re-solder the wires, I noticed that the phone works for as long the solder is hot, for a few seconds. No sound after it gets cold.
Trying to make sure if the glue-like thing is the reason for the above problem, I pried open the other, operational earbud and scratched the glue from it, as well. The same thing happened - the bud is not working, until I warm the solder. What is this glue thing? What can I do to substitute or work around it?

Thank you in advance for any of your answers

Photos of the PCB:

* Look for a hairline fracture somewhere. Perhaps heat causes it to close. Then when cool it opens up and loses contact. Or as another possibility, cooling causes wires to cross and create a short circuit.

* Test conductivity of the yellow substance. It may be an adhesive used instead of solder, because it is easy to apply to tiny fragile parts. To do your own repairs there is the conductive 'liquid silver' pen, or silver epoxy.