There's something ominous about Fitzgerald's choice of words here. "Flow" implies a continuous process, one that will stretch on into the future. The flow may be distant now, but that won't always be the case. Life will go on, and as it does so, the flow of thunder will only get stronger and stronger. There is an obvious foreshadowing here. There are serious troubles ahead for all the main characters—tragedy, conflict, emotional upheaval, and death.
Although you probably realize it, anyone coming across this line for the first time needs to keep in mind that "the Sound" is Long Island Sound: that is, a large body of water between Long Island and the mainland, rather than the ordinary word meaning "something that you hear."