Reversed phase in audio can happen when audio devices save audio in the wrong way, resulting in a bad audio, notwithstanding everything seems to sound fine with the headphones. In my case, with the Sony HDR FX1 and a shotgun mic from Rode, one channel of the audio is each time reversed. A reversed phase audio creates problems on certain devices. For instance, if you play back that audio in mono, or on a PA system, the audio doesn’t play properly anymore(garbled, low volume sound).
Sound editors and musicians often hear it immediately what is wrong, but for regular users it’s harder to notice.
For instance, You don’t hear it easily from a computer, unless you have a high end sound card connected to good quality speakers. A tell tale could be that the sounds appears to be thin, no bass, etc. But if you record a talking voice, it’s hard to discover this problem.
Luckily, there is a good way to find out, using Audacity, or any other audio editor. When you open your audio in the editor, a good audio will show you something like this:
As you can see, the wave pattern of this talking audio looks more or less the same, with some small variations.
This is fine. Below you see the same audio with a reversed phase:
Now you see that the channel under the first is vertically flipped. It is almost a mirror of the channel above.
Another way to discover whether an audio has a reversed phase is by saving the audio separately and convert it to mono. If then the audio doesn’t play properly, one of the channels of the audio is reversed. Most audio editors have a function to reverse the effect in a straightforward way. Cubase, calls it Phase Reverse, while others might call it Invert. However, they all have their own way to deal with the problem, therefore best look into the help section of the editor.
What is an audio phase and what causes a reversed phase?
It can be that the mic or mixer is badly wired up, it is surprising how often that happens, actually. Or you changed polarity by accident. With musical instruments, there are other factors involved. Here you find an excellent article that describes in details what a reversed phase is and some ideas how to avoid it, especially with more than one instrument: http://www.uaudio.com/blog/understanding-audio-phase/ by Universal Audio.