Q: In FCPx how do you invert the phase of one channel of audio?

Apply the Audio Gain effect to the clip/clips. Open the Parameters section of the Gain effect in the inspector. You'll see a Phase Left and Phase Right toggle switch. Toggle either one of them.

However, you shouldn't be getting complete cancellation of the audio, unless it's in mono. Even if it was in mono, if the phone has stereo speakers, it shouldn't cancel out completely. Sounds like something else is up.

Open your Effects window (Command-5). On the left on that window, select the "Levels" subsection under Audio. The Gain effect is about half way down the list in the right side of the window.

IF the problem is inverse phase, all you need to do is switch either the left or right, which will put whichever one you switch back in phase with the other one.

HOWEVER, as a test, if you hit the "Make Mono" button in the Gain effect BEFORE switching the phase of either channel, it will combine the two channels. If they completely cancel out during playback, phase was your problem with the phones. If it doesn't cancel out though, your problem isn't phase. Again, I doubt a complete lack of audio is due to phase issues, but test it out.

On a more technical level, neither channel is actually out of phase when people talk about two channels being 180 degrees out of phase. One of the channel's polarity is reversed, meaning the voltage, bits,  or whatever, is increasing when it should be decreasing and decreasing when it should be increasing. The audio channel is doing the mirror image of what it's supposed to be doing. Being out of phase actually refers to one of the channels being moved in time in relation to the other channel, which is a different thing entirely.

Oh, and the slider is just for gain (volume) of the audio. Unrelated to your goal to affect phase. Leave it alone.

No idea. So many variables. If you can hear the audio fine on your computer while editing, and hear it fine on the final output file when watched on your computer, and it's only on the phone that you're not getting audio, it's the phone/s, most likely.

Unless the speakers are broken, it's probably an issue with the file you gave it. What kind of file (wrapper, codec) are you asking the phone to play? I'm guessing an Android phone (though I don't know) should be fine with an h264 video with AAC audio, in an MP4 wrapper. Even if the general format is ok though, maybe it's encoded at too high a bitrate for the phone to play back. The phones having a problem with the format sounds like the most likely scenario. But try your phase inversed audio file. Who knows? Maybe that will do it.

It will invert it when you actually output the file. It doesn't do anything to it immediately.

AAC audio is one of the options for audio format in an MP4 file.

I came upon this post just yesterday when researching a problem, and while hopefully you're long since discovered what you need to do, I figured I would post a reply for anyone else who, like me, stumbles upon this thread when trying to diagnose and fix mobile audio issues.

The advice in other replies about adding the Gain effect in FCPX and using the setting to invert phase is a great suggestion, and it could certainly do the trick if the Phase Cancellation is happening on two independent audio recordings, but inverting the phase didn't work for me and I'd like to share what I discovered in case anyone else has the same issue.

In my case, I had a single video clip of an interview with two speakers and I added stock audio loops as a background track. The audio sounded great when played back on my computer and when played from a browser window on YouTube and Vimeo. However, when played from m.youtube.com or the Vimeo app on a phone, it sounded terrible:

• The interviewer and interviewee from the video were not able to be heard, but garbled pieces of the background noise did come through.
• The volume levels of the background music were significantly louder, even though I added keyframes to reduce the volume when the interview started.

After scratching my head and troubleshooting a bit, I found that Phase Cancellation was indeed the problem, but it wasn't a matter of cancellation between the music and the interview tracks. The issue was happening within the audio track that was imported along with the video as an .MTS file. The .MTS file records a separate Left and Right channel, and they were out of phase with each other.

As I received the video from a client, I was unable to troubleshoot further on the video camera to find out if this was across the board for all recorded videos or something about the way this particular video was recorded or exported, but in order to quickly fix the issue without reworking any of the post-production effects, here's what I did:

1. Select the audio track in the timeline
2. In the "Audio" tab of the inspector, change the "Channel Configuration" from Stereo to Dual Mono
3. Uncheck one of the Mono channels to mute it
4. Replay the audio in the Timeline to confirm that the overall project isn't negatively impacted by muting one of the channels. You can switch the checkboxes between the two mono channels and pick the best one. Luckily in my case, they were nearly identical.
5. Export in Stereo as usual

While I've done a lot of post-production of video tutorials from screen captures, I am less experienced in working with live on-site recordings, so am by no means an expert in this particular arena, but I hope this might help you avoid headaches in the future if you experience phase cancellation within the same audio clip!