How to get sound on Skype !

The first thing you should do is check that your microphone is connected and turned on.

https://support.skype.com/en/faq/FA10885/why-can-t-my-friend-hear-me-on-skype-for-mac-os-x

If your microphone is connected properly but you're still having audio problems, you might need to check your Sound Preferences on your Mac, and the Audio settings in Skype. If you are using a headset, the audio settings should default to that.

Ensure the correct sound input device is selected for your Mac:

1. In the menu bar, click the Apple logo > System Preferences… and then click Sound.
2. On the Sound panel, select the Input tab. 
   1. In the Select a device for sound input option box, select the appropriate device.
   2. Start speaking into the microphone, and in the Settings for the selected devicesection, adjust the Input volume slider until the Input level indicator registers in the upper limit.

Ensure the correct sound input device is selected in Skype:

1. Sign in to Skype.
2. In the main menu, go to Skype > Preferences… and then click Audio/Video.
3. On the Audio/Video panel, ensure the correct device is selected in the Microphone drop-down box and the input level indicator registers sound If it doesn’t, follow the instructions above to set the input volume on your Mac.

The problem could be with your friend’s speaker settings, so they should check their settings on their Mac and in Skype. Ask your friend to try the following:

Ensure the correct sound output device is selected for Mac:

1. In the menu bar, click the Apple logo > System Preferences… and then click Sound.
2. On the Sound panel, click the Output tab.
3. In the Select a device for sound output option box, select the appropriate device.
   1. In the Settings for the selected device, ensure the Balance slider is in the middle.
   2. In the Output volume section, ensure the slider is set to an audible level and Muteis un-ticked.

Ensure the correct sound output device is selected for Skype:

1. Sign in to Skype.
2. In the main menu, go to Skype > Preferences… and then click Audio/Video.
3. On the Audio/Video panel, ensure the correct device is selected in the Speakers drop-down option.