The issue was a dead left speaker. Everything else was fine. Curiously, it got back to life after poking on the speaker wires. But it died again after a few hours of use. From this behavior, I assumed that the speaker itself was not the problem. Instead, I blamed the wires. You see, what was used to connect the speaker to the PCB is a 2 stranded wire combined in a single insulated cable. The only insulation that keeps the 2 wires from shorting together is their enamel coating. My theory is that from long term use and vibration, the enamel worn out and the speaker shorted. It would explain why poking the wire temporarily fixed the issue.

Anyway, to prove my point, I got rid of those problematic wires and soldered 2 separated ones. Those are separated and have their own insulation.