First of all, if you haven’t already watched it then I strongly recommend – pardon my ego – my video about improving sound on the G25, as the defaults really aren’t that good in fact: http://www.youtube.com/watch?v=XVHNe-6runQ
As regards your specific problem, here’s your issue: the output of the radio mic receiver is BALANCED MONO but the camcorder input is UNBALANCED STEREO. Your 3.5mm lead is the problem.
In short, your lapel mic is mono. Being a “pro” bit of kit, it uses a system called balancing to help minimise any unwanted noise pickup between the receiver and the camcorder.
In balancing, the radio mic receiver takes your mono sound and makes an equal but out of phase copy. These are sent down the Left and Right sides of the 3.5mm jack and should only be plugged into equipment that is expecting a mono, balanced signal (which the camcorder is not).
The idea is that at the camcorder end, it would re-flip the balanced side and use this to help cut out any crackles and bangs. Since the camcorder is not balanced, it doesn’t know to do this and simply records the two signals as the Left and Right halves of the stereo recording.
When you play that back on the iphone, what you are hearing is the out of phase signals being mixed together back into mono and cancelling each other out. This is because the iphone’s loudspeaker is mono.
The reason you don’t hear it in headphones or on any other device is because they play the sound back in stereo so the sounds aren’t mixed and don’t suffer the problem.
You can probably check this in Logic Pro (or on your loudspeaker amplifier, if you use one) by switching your monitoring to mono. It should exhibit the same effect.
The “proper” solution is to use a different type of lead from radio mic receiver to camcorder. What you need is one that has one (and only one) of the stereo sides – it doesn’t matter which – shorted to the ground pin. That way, only one half of the balanced audio will be recorded (it will appear in the camcorder that only one side is working but this is fine for mono, which is what your lapel mic is)
Alternatively, you can buy a balanced-to-unbalanced adapter from companies such as Beachtek or Juiced Link and this will convert the signal correctly.
The fix for your existing recordings is to mute one side (either Left or Right, it doesn’t matter which) and pan the other one so it’s in the middle of your final stereo mix.
I hope this made sense and helps!