Hello, Juneau. 무슨 소리야, directly means 'what is that sound?', also you can use this as the meaning of 'what do you mean?' to far younger people or friends. But it can sound kinda rude to strangers or older listener. so the formal/polite form for older listener can be "무슨 뜻이에요?" or "무슨 말씀이세요?" (in the most polite language).

Correct, this means "what do you mean" A more polite way of saying this is: "무슨말 인지"