Audio scrambled on mobile from iMovie11

So I'm using iMovie 11 (9.0.9)

I'm using a Rode NTG2 to record sound on a Canon DSLR. All sounds and looks great when I import it all.

When I export to any of the file formats from the Export to Quicktime option, it all works fine viewing as a file on the computer.

When I upload it to YouTube it plays fine on desktop and tablet.

However when I view it as an embedded link on Facebook, the audio is "scrambled". Similarly, if I view it on YouTube on my iPhone 6 it is also scrambled. Unless that is, if I listen to it on earphones on the iPhone.

An example of what I mean is here: https://www.youtube.com/watch?v=tZNmGQmfY-s

So:

desktop: fine

iPad: fine

Embedded: not fine

Iphone external speaker: not fine

iPhone earphones: fine

I also use a Rode videopro (the type which sits in the cradle on top the camera). The sound isn't as good, but the sound works fine across all.

I've spoken to Rode who assure me it's nothing to do with the mic, even though there's clearly a difference.

Is it something to do with iPhones not playing "high resolution sound"? If so, how do I make the sound "low resolution"?

Thank you in advance for your help. This is driving me crazy...