There could be 3 reasons for this:

  1. MOST COMMON CAUSE: You are using an external USB microphone, e.g. Bluemic Yeti, and it sets itself as the default speakers. Plug in your earphones into the microphone (NOT your computer) to check if you hear any sound then! Read more info here for Windows or here for Mac  
     
  2. You have recently used recording software to record something on your device. In this case make sure to have the correct Sound output device selected.
     
  3. The other person is not sending any audio. In this case help them with these steps


Windows: how to fix no speaker sound with external microphone attached

This is likely to happen if you use an external microphone with output to plug in earphones.

  1. Before anything else, plug in your earphones into your external microphone to check that you hear sound
  2. Do you hear sound now? If not, then it's likely to be another reason.
  3. If you hear sound but want to get it through your internal or external speakers instead of earphones, here is how to fix it:

Step by step on Windows 8 or 10:

  1. Move your mouse to the lower-right side of the screen to open the Windows Charms Bar menu, choose Settings, select Control Panel, and click on Hardware and Sound.
  2. Click on Sounds.
  3. Select the Playback tab
  4. If you see your external microphone as Default Device with a green check mark, then you need to change this back to your internal or external speakers
  5. Select the other entry (your speakers) and click the Set Default button
  6. If you don't hear sound coming out of your speakers now, try to first refresh your browser window with your lesson. If that doesn't work you might have to restart your browser or computer.
Steps on Windows 7:
 
  1. Under Start Menu open Control Panel, click on Hardware and Sound, then select Sound.
  2. follow steps 3-6 above

Mac: how to fix no speaker sound with external microphone attached

This is likely to happen if you use an external microphone with output to plug in earphones.

  1. Before anything else, plug in your earphones into your external microphone to check that you hear sound
  2. Do you hear sound now? If not, then it's likely to be another reason.
  3. If you hear sound but want to get it through your internal or external speakers instead of earphones, here is how to fix it:

Step by step:

  1. Go to your System Preferences
  2. Go to Sound
  3. In the Output tab, make sure that your Internal Speakers / Built-in are selected and highlighted as default