Check you settings in two places for the correct audio out settings:
1. Look up under the System Preferences menu (the Apple pull down menu) the sound icon (looks like a speaker) and click on it to open it. At the top it will show three choices, Sound Effect, Output, Input. The problem you can't hear the Skype caller is likely the Output setting is incorrect. For example, you may have Line Out selected, but you are listing on head phones. Make the device name match what you are using to make sound.
Also check to see that the output volume is not set too low. Move the slider around to increase the volume.
2. Then check your Skype setting.Under Skype>preferences, select Auto/Video. Check to see what is selected under "Speakers: and check again to see the Volume is turned up high enough to hear.
Also make sure you have a good speaker or headphone connection.
You can check to see if you can hear by calling the Skype test call number to record yourself and listen to it. Go the www.skype.com/go/help
for more information.