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