Basics of waveform, how to define the zero point Audio is a sequence of waveforms which have peaks, troughs, 0, just like below.


How to define the x axis (y = 0) in the picture, how to choose the 0 point in the waveform since there are so many points on the waveform?

Why not move the waveform above or below the x axis(y=0)?

This question seems a little stupid. But I still hope someone can help with it.
You can't produce a DC component with speech, so the line $y=0$ is defined by the speech signal having zero mean.