[guineapig]

By clicking on any song icon it cancels whatever songs you are playing.
When you icons are not greyed out that means you are currently not singing anything.

For good measure just click your health song afterwards. That way you also have a visual queue in your song window. Alternately you can create a specific audio queue for whatever detrimental aoe song you are using that worries you. This way you can hear a little chime when your aoe dot ends.