The major problem is with the upper microphone (speaker), you must talk at loud close to this microphone to get heard.
When you record videos in vertical position the phone use the upper (bad) microphone so you can't hear nothing, there is a sound only when you scream near this microphone but still very very low, you could barely hear.
I've found that the stock Camera app that comes with the device (not the lumia cameras) works better with sound on videos on vertical, even though its still low.
It is a hardware issue - this microphone made sucks, its too damn low, but also a software issue, as at some applications you can hear better.
Also try to use Speaker mode during calls, the other side barely hear you if you won't speak close to this shi-tty microphone.
Microsoft / Nokia forgot its firstly a damn phone, to speak with people, then a camera/camcorder, these both have a major fail because of this issue of the suck upper microphone. Also they not gonna fix this or other issues (blinking/flickering).
This is the last WP.