For the most obvious difference, if you apply two waves to the same chord with the same phase but opposite frequency, the result is a chord which doesn't move (destructive interference). If you collide an electron and a positron, the result is an explosion with the maximum possible energy for their mass (mc²).
Also, you can actually do the phase cancelation experiment with two electrons, as they can behave as waves, and you will get the same cancelation as you do with audio waves - except the net electrical field will be 0 instead of the string not moving.
Comments
Is it not exactly like phase cancellation in audio engineering?
No, it is not.
For the most obvious difference, if you apply two waves to the same chord with the same phase but opposite frequency, the result is a chord which doesn't move (destructive interference). If you collide an electron and a positron, the result is an explosion with the maximum possible energy for their mass (mc²).
Also, you can actually do the phase cancelation experiment with two electrons, as they can behave as waves, and you will get the same cancelation as you do with audio waves - except the net electrical field will be 0 instead of the string not moving.