Q: My iPhone 6s, mute automatically and unmute automatically after few momoents. Need solution for this problem?

Is the mute switch damaged? Does it make a difference which position the switch is in? Or are you talking about voice mute when you're on a call?

tt2

Is it the same function as moving the side switch, or something different? Even brand new devices can occasionally have hardware problems.

tt2

How can you be so confident that the hardware is OK? There are lots of things internally that can still go wrong.  If you really believe it is not hardware, then the only advice I have is a restart, forced restart, restore form backup, and finally restore as new - in that order:

Restart your iPhone, iPad, or iPod touch - Apple Support

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

How to erase your iOS device and then set it up as a new device or restore it from backups - Apple Support

However, the best solution may be to make a reservation at the Genius Bar in an Apple Store near you or contact Apple Support.  Either way, here is the link:

http://www.apple.com/retail/geniusbar/

Please answer the question. Is the message you get on screen when "obverse that mute and unmute automatically which is showing on screen" the same as the message you get if you manually toggle the side switch or is it something else? If it is the same then my guess would be an intermittent hardware fault.

Either way this sounds like one to discuss with Apple Support directly. This is a user to user support forum. If you want to discuss the issue with Apple techs start at iPhone - Official Apple Support and scroll down to the Get Support option

tt2