When I plug my MacBook into an external audio source via the headphone-jack using the red and white cables (do not know the technical name), I only receive sound out of one speaker. At first I thought one of my stereo speakers had blown out, but I plugged my computer into another stereo using another cord and it still only plays one speaker. Also, when I plug in any set of headphones I only still receive sound through one speaker (the left one). My computers speakers still work, and when nothing is plugged in it still says under the output type that it's Built-in Output. I also noticed that sometimes (but not every time) when I unplug something from the headphone jack that a red light flashes around two times, but it does not stay lit when there is nothing plugged in. **Another note: The sound is not in mono in one speaker, it is just like one speaker is turned off.