Assonance is the use of repetition of vowel sounds. Perhaps the best way to illustrate this is by giving an example.
"Twisting twine twice is best." The vowel that is emphasized is the long "i". Also just to clarify, the "tw" consonant blend repetition is called alliteration, which is something different.
Consonance is the repetition of consonant sounds in the middle of a poem or at the end of lines. Here is an example:
"One upon a midnight dreary
While I pondered weak and weary."
The words, "weak" and "weary" form the consonance.
Now as to whether they can happen in the middle of the sentence, the answer should be clear - yes.