I'm almost guaranteed to be wearing headphones while at work. I like music. However, when I leave my desk, I rarely pause mplayer. This leaves my headphones leaking out some barely audible nois that may annoy coworkers.

I always lock my workstation when I'm not at my desk. How do I automate a solution here?

xscreensaver lets you watch the state of the screensaver. Let's use this to pause mplayer when I leave, and unpause it when I return.

xscreensaver-command -watch \
  | while read a; do
    echo "$a" | grep '^LOCK' && pkill -STOP mplayer
    echo "$a" | grep '^UNBLANK' && pkill -CONT mplayer
  done

Running the above, mplayer gets suspended when I lock my workstation, and resumed when I unlock it.