One important literary element to use for figuring out voice is tone. Tone is the attitude or mood that an author/play write conveys through his/her choice of words. The tone of a work or passage can always be characterized by one emotive word. Some words that characterize tone are ironic, playful, condescending, contemptuous, angry, or indignant. Thus, by analyzing Oberon's words, we can discover his tone and better hear his voice at this moment in the play.
One thing we can notice in Oberon's lines addressing Puck about the mix-up is that Oberon does not use harsh language, nor are there any exclamation marks. One example of his language can be seen in the lines:
What hast thou done? Thou hast mistaken quite,
And laid the love-juice on some true-love's sight.
Of thy misprision must perforce ensue
Some true love turn'd, and not a false turn'd true. (III.ii.89-90)
One thing we can notice in these lines is that Oberon addresses Puck with a question mark rather than an exclamation mark, showing us that Oberon is surprised by Puck's mistake, but not necessarily furious. Also, Oberon's next lines that speak of the lovers shows us that he is primarily concerned about the lovers' state. Oberon's heart genuinely went out to Helena, and he genuinely wants to remedy her situation. Now that Puck has goofed things up, Oberon is frustrated.
Later in the scene, Oberon's surprise at Puck's mistake turns to one of annoyance. We see his annoyance when we see him accuse Puck of meddling with the couples intentionally, as we see in his lines, "Still thou mistakest, / Or else committ'st thy knaveries willfully" (361-362).
Hence, now that we know that Oberon's tone is surprised, but not furious and turns to annoyance, we should better be able to write dialogue for Oberon telling Puck how annoyed he is. One example for beginning the dialogue, may be something like, "Puck, I know all about the pranks you like to play, but I'm surprised that you would willfully break up a couple by anointing the wrong man."