I just experimented with making a little video using iPhones and without having to compromise the sound.
The iPhone 6s has a very good video capability (especially when using the back camera (the 'non-selfie' one). The sound is not great, though.
I did a bit research and found a solution:
Use your iPhone with an external USB audio interface.
Here's what you need:
1. An USB audio interface with one or two mic pres and one or two mics.
2. If the interface is bus powered as mine you will need a USB HUB that has a power supply (I have a Focusrite Saffire 6 MKII as my computer sound card and it is bus powered, so I had to find a cheap externally powered USB 2 hub, which is very cheap these days.
3. For iPhone 6s you need an USB3 to lightning adopter with additional power supply lightning input, so that you can charge your iPhone during the filming and recording (uses a lot of battery power).
Connection: Mic(s) -> pres/audio-interface -> USB hub (connected to the power supply) -> USB3 to lightning adopter (connected to a power suply, too) -> iPhone.
For just filming the generic iPhone Camera app si very good. For editing I used an app called vivavideo and it works quite nicely from the import fom the camera roll to publishing in Youtube.
Here's my first result: an improvisation on my newly acquired Juan M Álvarez flamenca blanca:
This one is using the selfie camera though, which is not as good as the other one
Music is a big continent with different landscapes and corners. Some of them I do visit frequently, some from time to time and some I know from hearsay only ...
My Youtube Channel is: TheMusicalEvents