This sentence is commonly used when ordered to do something you do not have any intention of doing or when told something you weren't expecting and you get agry by it. The "b" can stand for anything you want it to, like: bitch, baby, boo, boy, etc.
Teacher: you are failing
person 1: what? how u sound b?
person 1: yo do my homework
person 2: how u sound b? do it yourself