This is a very strange problem and might be hard to explain.
I recorded a clip with audio from a Audio-Technica AT835b Boom mic on GH2 camera.
The audio sounds perfectly fine playing back in the edit. It sounds perfectly fine on Youtube.
However, when played on Youtube for mobile devices ( iphone or android phone), the voice from the mic is gone.
Here is the clip itself: Watch on mobile device and you will see you cant hear the clip from the boom at all:
http://www.youtube.com/watch?v=6-tgiLLbARw

Ive done tests and you can hear any sound effects and music under the boom audio but the audio from the boom mic is completely muted as if its missing!!!
Ive done tests. Ive tried converting the boom audio to WAV and same problem.

It comes down to the recording from the AT835b mic itself. You see, when using the on board mic, the audio is"hearable" as it should on mobile youtube. Switch and plug in the boom mic and its gone again.

Is there something in this particular AT835b mics recording, when used in conjunction with GH2, that alters an audio signal?

Easy the shotgun mic has 180 deg phase error in it..... when you listen on the PC have you noticed the sound image sounds VERY wide?
So in mono (on iphone) it will cancel out.

So the question is, what is the work flow set up to get that sound?
Are your cables in phase... pin 1 to pin 1, pin2 to pin 2, pin3 to pin 3?
Is there a Y split cable used anywhere?
Are you using an XLR to 3.5mm (mono) plug or a 3.5mm stereo plug wired incorrectly?
I bet you are using an over the counter XLR- 3.5mm adaptor.

This is the info you will need to get the correct connections...
http://www.rcrowley.com/CamAdapt.htm#value

Unfortunately that sound like the most common problem when adapting a XLR mic to work on a 3.5mm stereo input.

Your XLR adapter more than likely is wired like this:
XLR pin 1 - sleeve
XLR pin 2 - tip
XLR pin 3 - ring

the problem with this configuration is that "Balanced" XLR microphones send the main signal on pin 2 then sends the same signal on pin 3 but this at 180 degree out of phase. it does this as part of the way it cancels any interference it may pick up along the way.

And what you are experiencing is because you are sending the (+) signal to left and the (-) signal to the right channel, causing it to cancel itself when the signaled is summed to create mono.

what you need is to follow the steps outlined in the link that Bryan put in his response it will explain how the 3.5mm stereo connector needs to be wired to receive the mono XLR mic properly.

you can also fix this after the fact in your NLE by making your audio track from mic MONO by selecting only one side to be played back and basically killing the other (don't mix them or you'll get the same result)

Good luck

Yes, I concur that the problem is phase cancellation. Oddly enough the channels cancel in some modes, but not in others. I was able to make the effect come and go by switching the "HQ" mode.

1) You appear to be feeding opposite phases from the microphone into the Left and Right channels of your camcorder. This is almost certainly because of using the wrong cable/adapter. As Mr. Gallegos suggests, you probably have pin 2 connected to tip and pin 3 to ring (or vice-versa). As you have wonderfully demonstrated, that will NOT cut it on a consumer camcorder with a stereo mic input. As Mr. Reynolds, suggested, the diagrams on my website will show the proper way to take a balanced source (like your microphone) and feed an unbalanced input (like your camcorder).

2) YouTube appears to be playing games with the audio channels. There's not much we can do about that, but it wouldn't be a problem if you weren't giving them out-of-phase sound tracks. There are modes of stereo encoding where they combine L+R and send Mono and Difference (vs. Left and Right). For example, that is how FM stereo radio works. That is how they maintain mono compatibility. But when they combine your Left and Right tracks, it almost completely cancels out, so there is no Mono (L+(-R)) and no Difference signal either.

Suggest BOTH getting/making a properly-wired adapter cable, AND using only ONE of the tracks (Left or Right, but never BOTH) to eliminate this kind of problem.

You may be able to fix this in post, some audio programs and, iirc, Final Cut, let you invert one channel of a pair to fix this kind of problem. One such program is Audacity, excellent freeware. Click on the triangle tab on the stereo track header, select split to mono, select one track, select effects, select invert. http://audacity.sourceforge.net/

Thanks! FIXED IT!

All i had to do was apply a channel filter and make one Left Channel slightly lower and the right channel slightly higher - (just alter the channels slightly differently) and then it renders to a file which when played in MONO on a phone - is heard properly.

I don't understand phase cancellation but i guess that's what was going on... thanks for your helps.

Thanks again everyone, very knowledgeable answers here. And I learned something important from this experience. Eventually I plan to get a beach-tech adapter for my GH2 to run the XLR mic into that, and get rid of the flimsily XLR to Mini stereo to Micro stereo adapter cable im using.

Here's a video explaining a similar problem I had, and how I fixed it.

Fixing YouTube Audio Problems on Mobile Devices - YouTube