We all talk about the big bang as this moment the Universe begins by popping into existence with simple hydrogen and some helium. Hydrogen and Helium are gases that are able to carry sound. Today, space has expanded so much that there is vacume beyond where matter clumps together to form stars/planets, which means the sound couldn't propagate, but the early early Universe wouldn't have clumped like today; there would have been hydrogen everywhere. This means it would have been loud everywhere.

*Edit: I know we call it the Big Bang because of the rapid expansion of spacetime, but I always just pictured the general motion of expansion as the "bang". Never thought of how loud it would have been.