The speaker begins by listing out strong, seemingly everlasting substances: “brass,” “stone,” the "sea," and the "earth" itself. The speaker then laments the fact that even these materials are, in the end, temporary: “sad mortality,” the speaker says, will overcome—“o’ersway[]”—their “power.”
By calling mortality “sad,” the speaker communicates a sense of loss: time alters everything, these opening lines suggest, and even the most basic of materials—metal, rock, water, dirt—are subject to change and decay. Think about it: over time brass becomes tarnished, stone breaks down into sand, sea levels rise and fall, and earth is eroded by water and wind. All of these substances, in the speaker's estimation, are thus “mortal”—they are temporary.
The speaker uses polysyndeton (that repeated "nor") to add intensity and emphasis to this idea that nothing can escape "sad mortality"—not this nor that, nor that, nor that. The hissing sibilance here also makes the lines feel hushed and solemn, all those /s/ sounds bringing the speaker's voice down to a (perhaps sinister) whisper:
Since brass, nor stone, nor earth, nor boundless sea
But sad mortality o’ersways their power,
By connecting the words to one another through sound, this sibilance also subtly reinforces the fact that all of these things are subject to change.
This is a sonnet and, like most English-language sonnets, it uses the meter iambic pentameter—lines of five iambs, or feet with an unstressed-stressed beat pattern. Line 1 features perfect iambic pentameter, evoking the relentless march of time, but notice how things get funky in line 2:
Since brass, nor stone, nor earth, nor boundless sea
But sad mortality o’ersways their power,
While "power" technically has two syllables, it's likely that Shakespearean readers would have scanned it as having just one: "pow'r." A more obvious substitution pops up in the middle of the line, where there are two stressed beats in a row thanks to the word "o'ersways." Such disruption of the steady meter evokes time's destabilizing power.