This problem is usually caused by Skype overcompensating for what it believes to be loud background noise levels. Go into Skype’s Tools menu, then to Options… and select the Audio Settings tab. First, check that the HUE camera is selected as the microphone, and then uncheck the box below marked, “Let Skype adjust my audio settings”. Initially it is best to set the volume slider a little less than halfway along the bar. You can adjust this later to suit your preferences.
Remember to save your settings before closing the window. The sound levels should now remain steady when you’re making calls.