The way my Mac Pro always worked is that if I wanted to hear sound I just plugged my headphones in the front. When it was unplugged, no sound came out. This is how I need it because I'm using this at work. All of a sudden, when my headphones are unplugged I can hear the sound coming out of the box. I didn't even know it had an internal speaker. When I got to Sys Pref > Sound > Output, it says Headphones - Built-in Output when they are plugged in. When I unplug it it switches to Internal Speakers - Built-in Output. I can mute the internal speakers one but when I used to change the volume with the headphones unplugged it stayed muted rather then unmute. I don't know if I explained myself clearly but I just wanted it to go back to how I had it so I don't accidentally play something in the office.