Passion and slaughter, ruth, decay Descended, whispered grain by grain, Silted down swaying streams, to lay Foundation for our voicelessness.
Man, things sure get a whole lot darker this time of night. Look at that doozy of a list: "passion" paired with "slaughter," and "ruth" (sorrow) followed closely by "decay." Things were once pretty chillaxed and peaceful, but now all these bad vibes are figuratively coming down on our two lovers. Weren't we promised a positive night? This is anything but.
This list of negative forces comes down "grain by grain" (in a small way, like sand). These things are also whispering, which is still more articulate than the people who are unmurmuring and voiceless. It's like the speaker is saying that, even in moments of calm, he can still hear the voices, however quiet, of all the horrible things in the world. Bad times.
Oh, and in case you forgot about the extended metaphor of the ocean, here the speaker reminds you with some S word alliteration: "Silted down swaying streams" (23). All these bad things are like a kind of gritty backwash from the surf.
All this bad mojo "lay[s]" a "foundation" (which should make you think of the construction and architecture of lines 13-14) for the lovers' "voicelessness." They are struck dumb by all the terrible things in this world. Bummer.