It is important to make sure speakers are correctly connected to the amplifier, with the positive going to the positive and the negative going to the negative.  If set up right, then when a sound wave goes into the positive part of the cycle the speaker should push out, and as it goes into the negative part the speaker should pull in.  If your left speaker is wired backward so that a sound wave going into the positive would make the left speaker pull in rather than push out, then when that sound wave combines with the normal sound wave coming from the right speaker you would get phase cancellation.

Remember that polarity is the direction that a sound wave starts (into the positive or into the negative).  Is the electrical energy positive or negative?  The same principles that cause opposite polarity sound waves to cancel each other out are used to make particularly quiet audio cables.

Different audio cables are used for different purposes.  The cables used to connect a CD player are different from the cables used to connect a microphone, but they both carry sound through the entire length of the cable.  The longer the cable, the more a sound will weaken, just like the power behind a ball that is thrown a long distance is weaker at the end of the throw than at the beginning.

Weaker sounds need to be made louder, which unfortunately also means that whatever noise the wire picks up will also be made louder.  The problem is, how to make the wanted sound louder without taking the unwanted noise along for the ride?

The answer is kicking out the noise using phase cancellation.  To transmit sound through a cable you need a wire feeding electricity in and a second wire called a GROUND. Special cables called BALANCED cables have three wires. The first wire is the ground.  The second wire sends the sound along the cable. The third wire is connected backwards (phase reversed) but does not have any sound running through it.  Any noise that collects on the second wire (the HOT wire that has sound) will be out of phase with the same noise that has collected on the third wire (the COLD wire that has no sound).  Simply combine the hot and cold wires and you have the intended sound, but any noise collected on the way has been cancelled out.