Ok, close your eyes, and put on your imaginary X-ray vision. Set your speaker on the floor and stand over it, looking down. Watch the speaker cone, thru the cabinet, moving forward and back in it's metal frame. With your other eye, watch the waveform on the Sonar track. When the waveform goes up, the speaker goes forward. When the waveform goes down, the speaker moves backward.
This is a lie. It doesn't usually work that way, but the concept is useful.
If you had a very pure tone, like a flute, playing one note, with no harmonics or distortion, you still wouldn't be able to match the speaker movement to the waveform because the waves and speaker flutters are too fast at flute frequencies, but at very low frequencies, with a pure tone (sine wave), it would be theoretically possible, but we're talking ten cycles per second at frequencies too low for you to really hear, being still too fast to see.
So what? Simple wave goes up, speaker moves forward, wave goes down, speaker moves back. Write that down.
Now add another simple tone, same frequency. If the tops of the waves line up, the speaker moves twice as far forward. If the top of one wave lines up with the bottom of another, the speaker cone doesn't move at all, the two tones are fighting for control of the cone, the magnetic fields match, balance out, and nothing happens. You hear nothing, because a non-moving speaker moves zero air molecules. The two waves are 180 degrees out of phase.
Line the two waves up differently and something weird will happen. Part of the time the waves will add, other times the waves will cancel, and what you hear out of a speaker moving under those weird magnetic fields will sound weird too.
So much for the simple case.
Make some noise with a more complex instrument and it will generate a big pile of waves at a LOT of different frequencies. Do it twice, on two different tracks, in perfect sync to begin with. Add some delay on the second track, just enough to get....pick a frequency the 200 cycle per second waves 180 degrees out of phase. Now your speaker will NOT generate 200 hertz (cycles per second) audio, and stuff close to 200 htz will sound weird due to partial reinforcement and partial cancellation.
AND....
...you're going to see a similar dead space at 400 hz, with adjacent weirdness, more yet at 800 hz, and so on, AND a dead zone at 100 hz with still more weirdness at 50 ht to boot.
This is called comb filtering and it happens because waves that are out of phase at one frequency are also out of phase at exact multiples and whole number fractions of that primary frequency.
It's actually more intense that it sounds, because in between cancelling frequencies, other frequencies are reinforcing each other, so at 300 hz, give or take, the speaker moves MORE air than normal so the comb's teeth are even taller yet and the weirdness gets doubly out of hand.
Now don't freak out just yet.
Guitarists pay GOOD MONEY to do exactly this to their sound, by way of various devices called Phasers, and to some extent, Flangers, Chorus Modules, Delay, Reverb, even Fuzz Boxes and EQs. Just about ANY time you split a signal you are going to cause some frequencies to cancel and some to sum. Even if YOU don't split the signal, you room and speakers are going to without your permission. Some will audio energy will come right off the speakers direct to your ears and some will bounce off the ceiling and get there later, probably with less energy but it will still comb filter what you hear just the same.
In fact...you are going to have a real hard time getting totally free of phase issues...ever, even if you don't pay good money to do it on purpose.
But...first off, we as audio engineers don't just sit there and let the universe force us to do things a certain way, we are all about making sounds that people LIKE. If phase issues sound "good", we make em do it more. If not...we do...something else. And right there is the crux of the matter. Two of them.
First off, you have to realize that your ceiling, or EQ, or speakers, or E, all of the above, aren't likely to be the same as somebody else, listening to your genius on a different system, in a different room. And phase weirdness itself, is going to sum, unless you hit the lottery, and your room weirdness is exactly cancelled by the end listener's room weirdness, and dude, you actually have a way better chance of winning the Powerball than finding anti-perfect weirdness.
So you need to get all of the RANDOM weirdness out of your room, AND out of your system, so you aren't throwing your listener curveballs. And since that is half as unlikely as winning the lottery, since the best you can do is MINIMIZE phase issues in your imperfect audio/creative space, you also have to...
Second off...MINIMIZE the phase problems you can't ever totally get rid of, that hurt people's ears, and while you're at it, MAXIMIZE the phase weirdness peole LIKE, and pretty soon, Slash will be calling YOU to come fix HIS room.
Except, no two people like the same things exactly, but that's a whole other can of worms, and I'm gonna let YOU deal with that.
Basically...treat your room, connect your toys up with the polarity consistent from start to finish of each audio chain, have a mind towards what happens when you split a signa l(two mikes, a send and return with a mix control, a single piece of equipment that splits and then recombines the signal,what have you) and run a piece of it somewhere else before mixing them back together, learn to use your meters, a spectrum analyzer, phase meters, etc, understand that different groups of instruments filter EACH OTHER, sometimes in pleasing ways, other times like strep throat, three pack a day, dogs from hell, and finally....when a simple sound starts to remind you of a jet plane, whooshy and from another planet, look at your signal path and haul out your phase meter and figure out WHY your fighting wave reinforcement/cancellation.
Then and only then do you have the slightest chance of doing something about it to make things sound better.
Oh...and while you are attempting to beat the laws of nature, which Einstein pretty much proved can't be done, don't forget to have fun!