Oh, I don't know... Maybe with their vocal cords like most people do? Just a thought. :)
Is it possible to put 어떻게 in front or would that interrupt the sentence structure?
Is this question asking more like how physically the singer sings, like how they are able to ("how can they do that?"), or asking in what way it is they sing like; "are they good or bad," "do they have a high voice," etc?