The ground pins on each board are all connected internally, so the extra white wire you have is not necessary (as you alluded to) since you already have the black wire connecting the ground on each board.
Thank you for posting those pictures; it is a little difficult to tell, but it looks like you might be connecting to channel 0 on the Maestro instead of channel 1 (which is how you have the software set up).
If correcting that does not fix the problem, could you try removing the red wire from the Maestro signal pin and connecting it to a Maestro ground pin manually to make sure it triggers the MP3 track as expected?
Brandon