Lets say I have google chrome (or skype or any game such as starcraft etc.) open and I can hear the sound through my laptop speakers (Macbook Pro).
If I plug in my headset into the usb port or headphones into the headphone plug in, I am forced to restart my internet browser or whatever game it is etc.. Then I can get audio through the device I choose.
I've checked my default audio devices and reset them to whatever device I use at the time but it does no good (I also make sure that they are enabled).
**I used to be able to just have a song playing or whatever... then I could just plug in my headset or headphones and the audio would simply transfer to the new device.