Windows only: Portable utility Mute on Lock turns off your system speakers whenever you walk away and lock your workstation, but it also provides a quick shortcut key for toggling your speakers on or off.
Once you've downloaded and launched the no-installation-required utility, your speakers will turn off immediately after pressing the Win+L shortcut key to lock your workstation, and then turn back on once you've unlocked your PC. If you head into the settings page, you can set up a custom shortcut key to mute or unmute the sound, or customise what the tray icon does. Definitely a handy utility for those of us that lock our workstations religiously, and it doesn't take up a lot of resources either.
Mute on Lock is a free download for any version of Windows, though readers should note that you might have to use the Windows Compatibility Mode feature set to Windows XP to get it to work properly on Windows 7.