Do the 2 sound waves in the picture (same frequency) sound different?
This comes down to the question of audibiltiy of phase in a signal. To test this, I created two signals (both with f0 = 220.5 Hz, Fs = 44100 Hz) in Matlab, a saw wave with an upwards facing ramp and one with a downwards facing ramp:
ramp_up = repmat(linspace(-1, 1, 200), 1, 440)'; ramp_dn = repmat(linspace(1, -1, 200), 1, 440)';
These are (excerpts of) the resulting signals:
You can listen to them here (careful, relatively loud):
There is no audible difference between the two. Let's now look at the magnitude and phase spectra of the signals:
We can see that the magnitude spectrum (top part of graphs) for both signals is identical, yet there are differences in the phase spectrum (bottom part of graphs). For these signals, the difference in phase seems to be inaudible. Note however that current science on this topic suggests that under lab conditions phase distortions can be audible. Further reading on the topic:
They will sound exactly the same. The human ear responds to the frequency of the waveform - not the direction of the waveform itself.
One curve is the negative of the other. This is generally indistinguishable. However, playing one curve on one ear and the other on the other will likely be distinguishable from playing the same on both ears. I doubt that you could distinguish which of the curves (main part of the curve being pressure increase or pressure decrease) was which, though. Just whether one ear got to hear the inverse of the other or not.