The first blank should be filled with alliteration, as it refers to the repetition of initial sounds in closely placed words. The second blank should be filled with consonance, which describes the repetition of consonant sounds anywhere in several words within a line or short section of text.
So the complete sentences would read: Different poetic elements describe the repetition of sounds. Alliteration is heard when two or more words in a line or short section begin with the same sound. Consonance describes when the same consonant sound appears anywhere in several words in a line or short section of a text.