I have an older Macbook Pro. When I call someone on Skype, Google Hangouts, etc., they are able to hear all the audio that my computer is playing through the headphones. E.g. if I watch a video while Skyping someone, they can hear all the audio from that video through their own headphones. This applies to any sounds my computer makes (including the volume change sound, other system sounds, etc.). I originally thought it might be some sort of line interference from poor quality headphones, but tests with different headphones rule that out. Please, any thoughts on what might be causing this?