The voice John hears is so other-worldly that John is not able to pinpoint precisely what it sounds like; the best he can do is use a comparative: like.
As a prisoner on the small island of Patmos, he has undoubtedly heard the roar of waters crashing on the shore during a storm, even as he was also familiar with loud thunder crashes. And that, he says, is what it sounds like.
2 And I heard a voice from heaven like the roar of many waters and like the sound of loud thunder. The voice I heard was like the sound of harpists playing on their harps,