Sounds like the dogs aren't turning, you could try sliding in a putty knife and see if you can get them to turn, otherwise If you are going to be patching the wall anyway you may need to break out the scalpel:
Just took a quick glance at the manual, you will need to loosen the screws a bit, then push down (toward the wall) on the screw and down on the speaker so that the dog is not touching the back of the drywall then turn counter clockwise.

Just took a quick glance at the manual, you will need to loosen the screws a bit, then push down (toward the wall) on the screw and down on the speaker so that the dog is not touching the back of the drywall then turn counter clockwise.