@Zhw 

Hi,
You can refer to the following steps to troubleshoot.
    1. Make sure that there are no obstructions covering the microphone and speaker openings on the camera.
    2. Confirm the microphone and speaker volume of the camera are set at a high level on our app.
    3. Ensure the camera is positioned to allow clear and unobstructed sound reception. Avoid placing it in enclosed spaces or near surfaces that may reflect sound.
    4. Test the audio with your mobile device positioned away from the camera, preferably in a different room. The proximity between the phone and camera during a voice call can cause audio feedback.

If the problem remains, can you send us the videos showing the reported issue, and describe how you record the videos?

Best Regards