In Sonnet 130, Shakespeare compares certain physical features of his beloved with a number of beautiful things drawn from the natural world.

On the face of it, each comparison appears far from flattering, as the object of the speaker's love is considered less beautiful than the sun, the coral, the snow, and all the other features of the natural world with which the lady is compared.

The process continues later on in the poem, where the beloved's voice is compared to music and found wanting. The speaker loves to hear her voice, but he concedes “that music hath a far more pleasing sound.”

Not very flattering, it would seem. But what Shakespeare is doing here is to turn the conventions of love poetry on their head. His attitude is that, yes, my beloved may not be as beautiful as roses, red coral, and the sun, but so what? She'll always be beautiful to him. And it's certainly ridiculous to make a comparison between the human voice and music. They are simply not the same thing at all.

The metaphor of music drives home the point that the beauty of the speaker's mistress is very much in the here and now, whereas the beauty of music, like the beauties of nature, is transcendent and partakes of an ideal form of beauty.