the dreaded " Yellow Glue" problem has been mentioned here a few times.
I have exactly the same problem, but the glue is now black instead - with a KRK 'Rokit 5" powered monitor - from China of course. Both PCBs are liberally covered in the stuff, more on the amplifier and PSU one.
Symptoms in my example were loud crackling and low volume - which recovered when I wound the AC back to 160V !!
Seems you have to get every bit of it off to effect repair. Along the way you will see corroded leads and PCB link wires - there is a 2.2kohm next to the filter caps that is buried in the glue and needs replacing.
This is worse than the Yellow Glue problem as it becomes conductive in areas that have no heat.
Could the black colouring be carbon ??
... Phil