Aku Headcanon #7
Aku can change his voice at whim to imitate any person, living or dead, and he can also imitate any number of animal sounds.
Sometimes when he’s feeling particularly content and happy, he has a tendency to purr like a cat. He can also growl and bark like a dog, roar like a lion, hiss like a snake, and so on. The sounds come out purely on instinct; and it’s not something he consciously controls, most of the time.
That’s not to say that he can’t consciously control the sounds if he put his
mind to it, though– because he can.
[See S01E06 “Jack and the Warrior Woman,” when he changed his voice to sound like a woman’s. Thoughts on animal sounds are purely headcanon and speculation.]