Samsung 16-50mm Power Zoom Lens is noisy when using the power zoom buttons

I'll tell you what I tell everyone for recording audio for video, use an external mic and preferably an external recorder as well. That is how to get the best most reliable and predictable results. If you don't want to do that, then you might need to go for the 18-200 if you really need quiet zoom (it's pretty quiet as long as you don't zoom to fast) and AF. The 18-200 is made for video and uses a voice coil motor for extra quiet focusing. Still though, that lens is not good for stills and would cost you quite a bit more than a dedicated recorder.

There is no way to have good audio with build in camera mics. The mics are tiny, into camera, have no wind protection (it doesn't have to be windy for atmospheric pressure interfere with sound), have the wrong direction for recieving useful audio, whatever you do on camera affects the sound (press buttons, change dials, change your grip etc).
The mic input helps with adding a small microphone, that helps with some of these issues (better construction mic, above camera with specialized base plates, wind protection etc) but still, 95% of the time, this sound is good only as a lead.
As a 16 years pro sound recordist, my advice is that you can achieve much better sound, with the same equipment, just taking the main mic out of the camera, and into a stick!
For amateur use, in built mics are alright, for a more advanced hobbyist an external mic (you can't have that on NX500) or an external recorder (I have a few Tascams, Zoom brand seems always more orientated to music recordings, rather than video), for semi-pro/pro you need a sound recordist, the audio issues are far too complicated for inexperienced users if you want pro results, or "almost".