The function of onomatopoeia in songs is to reinforce a musical concept or theme addressed by the lyrics. The sound the word approximates may be repeated or duplicated instrumentally during the song, or the word itself may stand in for an instrument that the singer cannot readily obtain. These musical sounding words may also be paired with similar sounding words to complete rhyming couplets.
Onomatopoeia is a literary term applied to those words whose sound suggest their meaning. For example the word "hiss" makes a sound when spoken that mimics the action. Someone telling a story might say that he was startled by a snake hissing at him. Though the snake did not physically say the word "hiss," the storyteller can convey the similar sound the snake made when it warned away the intruder.
When used in poetry, onomatopoeia can add musical sounds to the spoken words. The lyrics of a song are often poetry set to music with deliberate rhythm. Onomatopoeia in songs can serve to reinforce any musical concept that is addressed by the lyrics.
In "Boom Boom Pow" by The Black Eyed Peas, the artist sings about moving through life to a specific type of rhythm. It is described as a "bass overload" (line 8) that belongs to a futuristic version of "rock and roll" (line 9). The artist places the reinvention of this new music in an era closer to "3008" (line 19) and says that it is the sound of a "future flow" (line 10). The overall theme of the song is the "Boom Boom Boom" futuristic beat, which is also the featured title, and is reinforced by the rhythmical sound of a bass drum that is played in unison with the singer.
Onomatopoeia in songs may also be used to substitute for a sound that the singer cannot reproduce, but can only approximate. The children's song "Are You Sleeping" references morning bells that call to the still slumbering Brother John. The bells are mimicked at the end of the song when the singer says "Ding Ding Dong" to reproduce the sound a bell might make, as it is unlikely the singer would be likely to ring an actual bell at that point in the song.
The word "Dong" that finishes the song additionally creates a slant rhyme with the word "John," both using the open vowel "o" sound, used in the previous line. In this way, onomatopoeia in songs can function to complete a rhyming couplet and to restate the theme. Similarly, in "Boom Boom Pow," the artist says that he is moving to a "supersonic boom" (line 27), and immediately likens it through rhyme and repetition to a "spaceship zoom" (line 28). The onomatopoeia used in these two lines reinforces the primary theme of the song, that this dance beat is the sound of the future, while comparing it to the equally futuristic sound a spaceship might make while flying through space.