There's no such thing as "the peak of an average waveform", since it doesn't make sense to average them.

They range from 20 Hz to 20000hz, so they have lengths between 2 cm and 1700 cm and peak everywhere in between.

What he was doing was attempting to avoid phase cancellation from stereo, which is an actual issue. You simply have to place yourself in the center, so that the distance to each speaker is the same.

It's an issue for low frequencies, which have long wavelengths, carry a lot of energy and are usually centered (to avoid phase issues in the first place), so it is both possible and audible if they cancel out after leaving the speakers. However, since they're long, it also means that there is some wiggle room. Obviously there's a perfect spot, but It won't have any noticable negative effect unless you're like 1 meter away from that and accidentally sit in the perfectly bad position. It is not as much of an issue for higher frequencies, since they have much shorter and more complex wavelengths, that are not necessarily centered in the first place. Even if they also theoretically do cancel out, it is unlikely that you would notice it happening at all, and if you do, you could simply move your head 1 cm to get into the right spot again.

In a car, the driver position is sort of fixed. It would make sense to angle the speakers for this purpose.