I had one who chirped once when her cage mate got out of the hospital cage I had her in after she had surgery. She was not supposed to be too active, so she spent a while in a smaller cage next to her cage mate. I must not have shut the door properly because I heard the one chirp, came to investigate and found the hospital cage open and empty with my convalescing pig right next to it. Had the healthy pig not chirped right then, we would have had a search to find her cage mate.
Earlier this year I was sick and spent the day at home. I put in some laundry early in the morning because it needed to be done, then went back upstairs to lay down again. The washer and dryer are in the basement, the piggy cage on the main level and the bedrooms upstairs. I woke up to hear a rhythmic high-pitched sound and thought at first maybe the washer was unbalanced, but realized that there's no way I could hear the washer from two floors away. No birds were singing and I didn't have any ceiling fans going. I can only conclude that one of my current pigs was chirping. I think it was probably Gabby because she is by far the most vocal of the two. She lives up to the name; we didn't know when we named her that she'd turn out to be so talkative.