I also Felt this. Lets wait for next update.
In a voting where 67 people voted 97% were facing this issue. Source:en.miui.com/thread-32078-1-1.html
It is very common. People are also getting replacement for this issue.
It looks like a software bug but still they are giving replacement. If you try "SECRET VIDEO RECORDER " app and set audio source to main or mic, the hissing greatly reduced.
If you search about it on Google you will see its happening in Malaysia, phillipene too.
This problem was there in nexus 5 too source :*productforums.google.com/forum/m/#!topic/nexus/0hvTK4eMVGk
It was solved by a xda member source :forum.xda-developers.com/google-nexus-5/general/mod-camcorder-audio-quality-fix-t2837930/
Please many of you have this phone tell me if you are having this problem. Is this a software issue or a hardware problem. Is it solved in any future update. I am getting that hissing sound while recording video. I received the phone yesterday.
Please help ASAP I am thinking of replacement or refund.