# symmetrical wav audio in time domain and Phase spectrum significance

I have plotted many wav files in MALTAB and tried to understand their time domain plot , Magnitude spectrum and Phase spectrum.

But I don't get why time domain plots of speech wav files are symmetrical around time axis. Why is it needed to be so ?

Here is the plot for speech :: The sky that morning was clear and bright blue.

I observed its Magnitude spectrum and got an idea about magnitudes of sinusoids components at various frequencies.

But What Information can be derived from its Phase spectrum relating it to time domain ? I have only understood that these are the phase values of sinusoids telling which is ahead or behind. But What else useful information can be obtained from the Phase spectrum ?