Is FaceTime audio and video really encrypted? So that we don’t have to worry about anyone listening to our calls when we call using FaceTime?
Absolutely it is!
On Apple's Approach to Privacy page, you will find the following statement under the iMessage and FaceTime section:
We use end-to-end encryption to protect your iMessage and FaceTime conversations across all your devices. With watchOS and iOS, your messages are encrypted on your device so that they can’t be accessed without your passcode. We designed iMessage and FaceTime so that there’s no way for us to decrypt your data when it’s in transit between devices. You can choose to automatically delete your messages from your device after 30 days or a year or to keep them on your device forever...
...and we never store the content of FaceTime calls on any servers.
Let me know if that was helpful or if you have any further questions regarding that statement.