Why is waveform asymmetric -- with different look on top than bottom?

This is quite normal. Happens a lot, especially with anything involving breath - like brass instruments and especially the human voice...

Basic explanation: What the waveform represents is the instantaneous vibrations of the air, as picked up by the microphone. By convention, positive pressure waves displace the diaphragm forwards, and this is represented by the waveform line moving upwwards. Because air is elastic, the pressure won't stay positive, and the waveform will revert back to the centre line. But because your voice is essentially a vibration of your vocal cords and they are elastic too, the forward movement of them will immediately be followed by a rearward movement which is a negative pressure change, with the waveform line moving downwards, and then back to the centre. If you ping a ruler over the edge of a table, you'll see the effect.

But, you will be asking, why the asymmetry then? This happens because when you speak or sing, you are breathing out. Rather than this being a vibration, the breath is only going in one direction - forwards. And that's going to displace the microphone diaphragm in a positive direction - so the waveform goes slightly higher up on the screen. It shows because the amount of air you breath out is a lot greater than the vibration displacement caused by your breath, and the diaphragm displaces a lot further. This is why we need, and should use, windshields with mics - they are primarily intended to diffuse the air blasts, and can often reduce the amount of asymmetry in the process.

If you see larger peaks going downwards, your mic is simply wired with a polarity inversion (often incorrectly described as 'out of phase'), and this doesn't matter - you can invert it again in Audition.