All of the DJ Pon-3 || MLP:FIM

Also known by the far superior name "Vinyl Scratch." Seriously, guys, we really missed the boat on making that her canon name.
Another quick video compiling all appearances of a background character, this time DJ Pon-3. It was actually a lot easier than compiling Derpy's appearances, for the simple reason that DJ Pon-3 is a background character with a specific use, unlike the likes of Derpy, Lyra, Bon Bon, etc, who can really appear anywhere.
Interesting note: She is shown to "speak" twice throughout her appearances in Friendship is Magic. The first time is at 0:39 where she is one of many ponies to say "shh!" to Pinkie (though "shh" is an unvoiced consonant sound so it doesn't necessarily hurt the head canon that she's mute). The next time is at 5:31 where she's clearly speaking to Octavia in the background, though it's only within the context of a fictional story, and is not something that the real DJ Pon-3 ever did.
So in regards to the question of whether she can speak or not; My conclusion? Inconclusive.