In the menu bar, click Skype > Preferences and then select Notifications.

Click the little speaker button to the left of Play sound. If you can hear a sound being played, your speakers are working correctly.

To adjust the volume of the speakers, go to the right of the menu bar of your computer and click the volume button. Use the slider to adjust your volume.

If you cannot hear any sound, make sure that the device you want to use as your speakers is selected in Skype. At the top of the panel, select Audio/Video. In the Speakers drop-down box, ensure the correct speaker device is selected.

Go back to the Notifications panel and try playing a sound again, or try making a test call.