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