You can say ...の音が近い. And it implies the distance between you and a sound source is close (in this case fireworks). 花火は近そうです explains you have the mental image of "the distance between youI and fireworks seems close to me" concluded by what you actually hear it or what you see it or what you are told by someone else and so on. After or before that, you can give more opinions by saying loud or something else.