Although I don't have piggies who chirp, I have piggies who sounds like a huge chourus of Piggies.
Around 10am, a little bit after I wake(About an hour, its usually earlier though when they start) I'll heard this loud, unusual wheeking. So I go into the room and they are nearly like standing side by side, wheeking as loud as they're lungs can wheek until they see the food in the bowl. THEN They stop wheeking. But when they wheek, it sounds like a chorus. Its loud, but its beautiful.
I think all piggies have a type of talent that some people have to discover, or help the piggie find. It took me 6 months to find Carly's voice. She wouldn't wheek when she was hungry or thirsty, she wouldn't walk around, staring at me. She'd just curl up and kind of stare at me for a second then look around my room. Then we moved Sandy(the loud one) into her cage and then she just began to discover her voice. Its like the people who gave her up(To Petco) just never listened to her or hated her voice so it made her scared that I did too. You never know your piggies talents unless you help them discover it or just wait and listen.