This is a very interesting question. If you are to write an introduction on sound devices in poetry, I would suggest you start by explaining how a poet usually wants to convey a message through their work. In order for this message to be fully understood, a poet may choose to use certain techniques in order to highlight his message further and ensure that the readers fully understand what the poet is trying to say. Sound devices are one aspect of these writing techniques.
You might then want to explain a few sound devices in more detail. There are several sound devices to choose from. For example, you could mention alliteration. Here, consecutive words start with the same letter, usually consonants. By employing this device, a poet might, for example, try to draw the reader’s attention to certain parts of the poem. An example for this can be found in the poem “The Raven” by Edgar Allan Poe: “While I nodded, nearly napping.”
Another sound device that you might want to mention is onomatopoeia. Here, the poet tries to recreate a sound through words. Again, Poe’s work provides a good example to illustrate this: in the poem “The Bells,” Poe writes that the bells “clang, and clash, and roar.” This choice of words clearly imitates the unpleasant sound that these bells are making, again helping the poet to stress his words further.