UPDATE - The recent update (FP6.QREL.15.122.0) considerably improved the quality of the audio recordings. I took several videos at live concerts, with drums, guitars and (moderately) loud amps, and it now works very well. It sounds like there is no more autogain or agressive noise gate being applied, and this makes the fairphone usable to record videos at concerts or rehearsals.
Thanks to the fairphone team for solving this audio issue!