Last night, again I hosted an online class. The guest was Syeikh Aiman. I invited some of my friends, alhamdulillah at least there were about 5 people joining the experiment class. I don't know why is the volume was soo low for all participants. Unfortunately, I couldn't hear any voice. Finally, I found that the problem was from the laptop that I used.
Make sure you try the microphone before you proceed the session.