When you say the amplitude of the total sound is ten times the amplitude of one of the sounds, you are assuming all sounds are in phase, in which case your answer is correct.

The question you are answering uses the words "in unison," but doesn't explain what that phrase means. If by "unison" they mean in phase, then you are correct. But if "unison" is supposed to mean "at the same time," the answer they give is correct if we assume that all the sounds are uncorrelated, as in white noise. In that case, we can add the energy of each of the separate sounds. The addition of the energy results in the addition of the Intensity, which is the energy flowing per unit time and per unit area.

When you say the amplitude of the total sound is ten times the amplitude of one of the sounds, you are assuming all sounds are in phase, in which case your answer is correct.

The question you are answering uses the words "in unison," but doesn't explain what that phrase means. If by "unison" they mean in phase, then you are correct. 

But if "unison" is supposed to mean "at the same time," the answer they give is correct if we assume that all the sounds are uncorrelated, as in white noise. In that case, we must add the energy of each of the separate sounds, because there will be just as much constructive as destructive interference among all the separate sounds. The addition of the energy results in the addition of the Intensity, which is the energy flowing per unit time and per unit area in the path of propagation.