private void SystemEvents_SessionSwitch(object sender, SessionSwitchEventArgs e) 
{ 
  //If the reason for the session switch is lock or unlock  //send the message to mute or unmute the system volume 
  if (e.Reason == SessionSwitchReason.SessionLock)   
  { 
    SendMessageW(this.Handle, WM_APPCOMMAND, this.Handle, (IntPtr)APPCOMMAND_VOLUME_MUTE); 
  } 
  else if (e.Reason == SessionSwitchReason.SessionUnlock) 
  { 
    SendMessageW(this.Handle, WM_APPCOMMAND, this.Handle, (IntPtr)APPCOMMAND_VOLUME_MUTE); 
  } 
}