Here are the variations I've tried:
Nothing (no mod, no speakers, no solder on board): no sound (obviously)
Only right speaker in left terminal: static noise
Only left speaker in left terminal: static noise
Only right speaker in right terminal: normal sound
Only left speaker in right terminal: normal sound
Both right and left speakers in their respective terminals: static from left speaker, normal sound from right
Speakers in opposite terminals: static from left speaker, normal sound from right
Sound mod on right speaker: works fine
Sound mod on left speaker: worked, but then stopped
In addition to the static noise, I've felt the speaker and it seems to be obscenely hot even after just a few seconds of use.
It is possible that there is some solder residue on the copper-colored ground area below the +5V/GND solder points on the motherboard, but I doubt that this is the cause of the problem.
When I first attempted this mod, I accidentally switched the +5V and the +speaker wires, so I may have blown something in the motherboard.
As always, advice is much appreciated