One of two possibilities of how Juliet "speaks yet she says nothing" is that she is speaking softly to herself and Romeo is unable to hear what she says. If that is so, the audience is not privy to her words either. The other possibility is that her eyes speak, as explained by Shakespeare-Navigators.com.
Romeo goes on to say that "her eye discourses; I will answer it." It is possible that Juliet at this point is standing at the window daydreaming, and Romeo, seeing something deep in her eyes, feels that she speaks through them. These lines are from Act II, scene ii, also known as the balcony scene.
In this scene, the two have just recently met and fallen for one another. Romeo has come back to see Juliet again. Juliet is thinking of Romeo, as the audience discovers when she actually does speak, so it is possible that Romeo is able to read certain words from her eyes.