Camera mode: point your phone's back camera at the monitor. Tap anywhere on the preview to place the watch box over the area you want monitored (e.g. the new-chat notification area). Use the box size slider to adjust the crop area. Keep the phone steady — propped against something works well.

iOS note: Safari pauses when the tab goes to the background. Keep the screen on and tab visible, or set alarm volume high enough to wake you.