The simple diagrams we see in textbooks such as the one below are – well – too simple. It is not possible for two waves to cancel everywhere. That would violate energy conservation. What happens instead is that the waves cancel in some places and add in others. For example, in a double slit interference pattern there are bright spots as well as dark spots. In the sound interference demonstration there are loud spots as well as quiet spots. This will always be the case. It has to be. The energy is not lost, it is redistributed to the bright (loud) spots.