Why is my macbook playing the audio output through the microphone / chat client?

[BigJameson]

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?

 

[BigJameson]

Am I allowed to bump this up? Desperately need some help fixing this

 

[satcomer]

It's an easy answer. On the Mac Book Pro the microphone is usually in the right speaker grill.

 

[BigJameson]

I should clarify. This issue happens when a headphone/microphone set is plugged in (like these). So, neither the laptop's built-in speakers or built-in mic are active.

 

[satcomer]

When you plug the headphone into your Mac did you go to System Preferences->Sound panel and select the cord's Microphone so the Mac selects it?

 

[BigJameson]

It automatically selects the cord's microphone, and the "internal mic" option disappears from the list.