When I play some music in my system with headphones on, I get some level of volume and when I plug it out, the volume either goes up or down. Why is that so?
Many devices (for example, iPods, iPhones, Mac OS X) will keep separate volume settings for different outputs. This allows you to have one level if it's going out the speakers, but make sure you don't blow out your ears when you put in headphones. It can also allow you to have your computer muted without headphones, but have the volume not muted when you plug in headphones, which is very useful in an office.