I've had the same cheap pair of earbud headphones at work for almost four years. Every day I bring 8 CDs to work and can often gauge my productivity (or at least the level of external distractions) by how many I listen to through the course of the day. As I'm lost in thought or plowing through code, I often chew on the wire leading to the right earbud.
Today, all that chewing has crossed a threshold. When I started listening to music, only the left channel came through. I found that I could usually jostle the wires a bit to get the right channel to come in, but if I left the wire hanging, my arms would soon bump it and the right would cut out again. But if I put the right wire in my mouth, when it cut out I could usually fiddle with it between my tongue and teeth to bring the music back. So the likely original source of the problem is also the easy solution to the problem!
On a related note, does anyone know how to set the balance of audio output using ALSA? GNOME doesn't seem to provide a setup view to do it and the ALSA command-line and ncurses interfaces don't seem to give me the ability to balance all the way to the left.