The Merriam-Webster Online Dictionary gives the first definition of "onomatopoeia" as:
: the naming of a thing or action by a vocal imitation of the sound associated with it (as buzz, hiss)
In Romeo and Juliet, Act 1, Scene 1, Benvolio tells Montague how the fiery Tybalt had gotten involved in the servants' fracas:
He swung about his head and cut the winds,
Who, nothing hurt withal, hissed him in scorn.
Also in Act 1, Scene 1 of Romeo and Juliet, we find this exchange where Benvolio asks Romeo who it is he loves:
Romeo: What, shall I groan and tell thee?
Benvolio: Groan? Why, no. But sadly tell me who.
"Hissed" and "Groan" are good examples of onomatopoeia.
Early in the play, Montague asks Benvolio to check in with Romeo and try to find out what's been bothering him so much. Benvolio says that he saw Romeo early that morning, before sunrise, walking in the sycamore grove. Lord Montague, Romeo's father, replies,
Many a morning hath he there been seen,With tears augmenting the fresh morning’s dew,
Adding to clouds more clouds with his deep sighs. (1.1.122-124)
I couldn't find my copy of the play, so I found a version of it online (linked below). Act IV Scene V has many references to sound. One onomatopoeia that I found interesting was Peter's quote:
"I'll re you, I'll fa you; do you note me?"
This is in reference to the solfege music notes. There are also many references to "silver sound" at the end of that scene.
In Act V Scene III, Paris says:
"Or, wanting that, with tears distill'd by moans: The obsequies that I for thee will keep..."
The moans would be the onomatopoeia in this scenario.