I traced down this loud sun theory, and it comes from a post from reddit of a guy who did the maths and obtained a volume level of 100dBA, although with one bold assumption, which is that the sound of the sun would propagate just as well as its light, which would absolutely not be true if there was an atmosphere between the sun and the earth. This reddit post has then been cited in a few articles. Sauce for anyone interested https://www.reddit.com/r/askscience/comments/33xuxu/comment/cqpsap8/