Hey aslam56,
Sounds like you're having some audio issues during FaceTime calls. We'd want things coming through clearly as well. We'll be happy to help.
Great job isolating the issue further. If you've been able to record a video and having no issues with your audio, this indicates that the issue isn't hardware related, but instead, it's very likely to be network related. Were you making your FaceTime call over cellular or Wi-Fi? If cellular, test over Wi-Fi and vice versa to isolate even further which network is causing the audio performance issues during FaceTime calls.
If cellular, try moving to an area with better reception and trying again, if on Wi-Fi, you could always try restarting your modem and router and testing things out further.
"If your cellular connection or Wi-Fi network are running slow, or other users are streaming audio or video on the same Wi-Fi network, you might have these issues:Connection alerts or unable to connectChoppy video callsBlack screenDropped calls
Make sure that both you and the person that you're calling are using a fast Wi-Fi or cellular connection. If you’re using Wi-Fi, FaceTime requires a broadband connection."
This information and more on troubleshooting FaceTime calls is available here: If you can't make a FaceTime call on your iPhone, iPad, or iPod touch
We hope this helps out. Take care!