How can we monitor audio of subject using headphone while recording video using iphone or samsung galaxy s4?
You could use an adapter to split the cell phone jack into XLR &amp; 3.5mm.  Then just plug in your XLR mic and headphones.

Or you could record double-system audio.  Get a second, purpose-built audio recording device like a Zoom, and monitor that while you shoot with your cell phone.  It's super easy to match up the audio again in most editing software nowadays.