All Might is the hero everyone deserves. Except when he isn't. In which case he is a scrawny looking man whose head seems too big for their body. This is due to the power of his One-For-All quirk, which body provides his body with the power it needs to be a true hero, while also taking its toll as well. Muscle Form All Might should have a booming deep-mid voice reflective of a classic Superman. True Form All Might is a different story, wherein his voice simply needs to heavily contrast his Muscle Form voice. Go wild with his True Form voice, though the particular voice I envision in my head is something gravelly and mid-range.