Snow:

The speaker, defying the conventions of traditional love poetry, claims his mistress's skin is not as white as snow.

Roses:

This may be the most significant symbol. Roses are a staple of romantic poetry. Women's cheeks and lips are often compared to red roses, since roses are associated with love, idealism, and sensuality. However, the speaker denies seeing any roses in his lover's cheeks, puncturing this cliche.

Music:

The speaker claims he loves to hear his mistress talk, but he would never compare her voice to music. To claim her voice is not musical places her ever more in the company of ordinary people.

In using all these symbols, Shakespeare's speaker is claiming his mistress is not an exceptional person by objective standards, but to him, she is everything. It does not matter that she is not perfect—he loves her.

The symbols Shakespeare uses in this poem serve to enhance the imagery he creates in describing everything his lady is not.  For example, he uses snow as a symbolic standard of a pure, pristine complexion, and his love, whose skin tone is "dun", does not measure up.  In a similar manner, Shakespeare uses the sun, roses, and music as symbolic ideals of the radiant eyes, rosy cheeks, and melodious voice that he would expect to find in a classic beauty, and again, his lady is lacking in these areas.  Shakespeare uses these symbols to create an image of the traditionally accepted measures of comeliness.  In a tone that is playful, tongue-in-cheek, and self-effacing in a way, he makes a comment on the importance of these measures, or perhaps on the foolishness of his own judgement.  Although his love is not a beauty, he loves her still.