@Stamati 

Hi,
On the camera's Live View page, if you tap the Talk or Voice Call feature, and then tap the icon at the right upper corner, you can see the camera's microphone and speaker volume. You can adjust the speaker volume to a high level for a try, and please set your phone's volume to a higher one and check if it helps.

Please help confirm the following information if the problem remains.
1. Where did you install the camera? Please send me a picture of the location if possible.

2. How far between the person and the camera when he/she is in front of the camera to hear the voice?

3. Please try to use a phone near the camera to record the voice from the camera's speaker, and go to the location where you hear the camera's voice usually, record the voice from the camera's speaker, and send me the audio recordings for further analysis.

Best Regards