When you plug the Mono Headset into the Mac in the first time, the OS automatically changes the Audio setting to mute.
This setting needs to be manually changed back to unmute in order to hear Audio. This setting will be saved in the memory so you should not need to change the setting again after the initial settings change.
You will also have to go to Skype >Preferences and select the Audio option. Change the Input and Output fields to the Everyman option on the dropdown menu (NOT the Built-in).