Hello everyone,
The problem is solved,i found out that the problem is with hikconnect app, i tried with different hikvision viewing apps (i.e.ivms 4500/hd,hi-lookvision and more)but finally installed ezviz app and loged in to watch playbacks (on LAN)both video and audio is smooth as butter.
(I would say hikconnect is good for live viewing but not for hd smooth playbacks though[or maybe only me facing this problem])
But a drawback with ezviz app is that i can't access 4 playbacks at once and is only able to watch one cam view at a time.
Anyway thanks to all.