Don't worry. You don't have to trash your phone just yet. The small piece of black plastic is merely there to support the screw on the left hand side of the speaker. Luckily, it's still held in place by another screw on the other side and the case doesn't leave much room to wiggle around in anyways.
So the small black triangle isn't absolutely essential. Good if you still have it, not the end of the world if you don't.