Not hearing the other person at the start of a call is a common problem among all video software. Here are a few things you can try:

Is everyone's speakers turned on? Can you usually hear sound?
Sometimes someone forgets to turn on their speakers, has mute on, or has headphones plugged in. Make sure that everyone can hear sound from their computers, use this online sound test to make sure sound can be heard from everyone's computer.

Did you give your browser permission to access the microphone?
Your browser needs your permission to access your microphone for a call. Make sure you gave your browser permission to use your camera and microphone. If on an iOS device make sure mic is allowed in privacy settings for the app. Settings > Privacy > Microphone: make sure app is allowed.

Is your microphone muted?
If you’re using a headset or a computer with a press-to-mute button, make sure the microphone is not muted. Also, has a mute button has well, make sure isn't muted either. 

Restart computer <-- THIS SOLVES MOST ISSUES
Sometimes other programs that uses your microphone (e.g. Skype) are still connected to your microphone and don't allow your browser to access your mic. Restarting your computer will clear this issue. 

Run a microphone test
If a computer restart does not help, run the following microphone test and email the results to  

Check Your Computer Settings
Your computer settings might be preventing your from sharing your microphone. Here's how to check your computer settings on Windows and Mac

Contact your microphone manufacturer
If you’re still having problems, contact your microphone’s manufacturer.

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