I've tested with a 19v laptop adapter and got the same results.
Never do this, extra 2V is high enough to make chips damage.
Give marked working voltage to chips.
You make faulty area larger, otherwise.
Now you need to check ALL chips if they are working or getting hot.
If chips get damaged, Vcc shorts to ground always as well as hot.
How to troubleshoot, see photos
Download 1KHz wave file and input to the speaker via smart phone.
Then follow copper foils to see where signals go
that are inputs where it may be pre-amp or filtering caps.
Oscilloscope will be a great help when troubleshooting audio equipment.
If you have it, use it without any hesitation.
If chips are OK, check their peripheral SMD components.
I will leave police station and meet Ada Wong again tonight .