If you're keeping track of a binary state (I.E. one of two possible states), you can use a Boolean variable, E.G.
Begin Boolean Compare : [lightsOn] Equals False
Play sound, '{VA_SOUNDS}\lights_on.mp3'
Else
Play sound, '{VA_SOUNDS}\lights_off.mp3'
End Condition
Set Boolean [lightsOn] to Toggle
Make sure to enable the "Evaluate 'Not Set' as false" option for the "Begin a Conditional (If Statement) Block" and "Set a True/False (Boolean) Value" actions.