Assonance is the repetition of vowel sounds. In a sense, the act of rhyming is a use of assonance. But assonance is usually cited when vowel sounds are repeated in a line. For example, in the second line, "brave day" repeats the long 'A' sound. In the 13th line, "Time's scythe" uses assonance and links the two words, thus emphasizing how quickly time passes en route to one's death.
In the third line, the violet is "past prime." This is an example of using plosives and alliteration with the repetition of the letter 'p.' Plosive consonants such as b, p, t, d, and k have sharp, stopping sounds. Here, the 'p' is used for emphasis about how the violet is in decline. Again, the effect is to startle the reader and make him/her aware of the quick passage of time. The plosive sounds happen quickly. This is useful in emphasizing the quick passage of each second.
Consonance is the repetition of consonant sounds. It is more broad than alliteration because alliteration only refers to repeating the first letter of successive words. So, "past prime" is alliteration and consonance. Alliteration is a kind of consonance. In this sonnet, Shakespeare relies mostly on alliteration ("green all girded") and ("Borne on the bier").
Shakespeare uses the imagery of nature in decay to stress the passage of time. He uses these images to illustrate how quickly a life passes. Since nothing can stop Time, the only way to live on is by having children ("breed").