Words that all end with a ment sound occurs in the same line of poem is an example of?

Here's why:

* Assonance is the repetition of vowel sounds within a line or phrase, creating a pleasing effect on the ear.

* Near rhyme (also called slant rhyme or half rhyme) occurs when words have similar but not identical sounds.

In your example, the words ending with the "ment" sound have similar vowel sounds but don't rhyme perfectly. This creates a subtle rhyming effect that adds to the musicality and memorability of the poem.