Have you ever heard what a giraffe sounds like?
I thought I heard Koichi outside, but it was a little bird's chirps.
I hear a puppy barking under the vending machine.
The chirp voice is a sound an animal makes. Sure, it works for birds (which chirp), but it also works with most other animals as well. So, we call this an animal cry.
The readings are those that you learned with the kanji. 声 gets changed to ごえ, though, so take note of that in your head. Perhaps you can imagine an animal that does a "GOEEEEE" cry.