Audio quality has turned out to be a pretty disappointing issue with the Hero5 camera's new waterproof design with the 3 microphones. Unfortunately with the current firmware on the camera there is very little control on how the microphones capture audio. If you search on YouTube there have actually been several cases of customers using the Hero5 in non-gimbal applications, mounted to a dashboard of a car or mounted in a housing in the wind. They are experiencing the same audio quality problems with the camera picking up excessive background noise.
Examples on YouTube: https://www.youtube.com/results?search_query=hero5+audio+issues
With the current firmware on the Hero5 Black and Hero5 Session you are actually supposed to be able adjust the Audio Processing setting in Pro tune to "Low" which is supposed to generate a separate .wav file on the SD card. This .wav file is supposed to contain 3 tracks in it one correlating to each microphone on the camera. With this file you are supposed to be able to mute each of the 3 microphone tracks individually (mute the one next to the motor) - however there is a bug in the current firmware as the Hero5 generates a .wav file with no content in it. We spoke with GoPro's engineering team and they confirmed this.
Unfortunately the camera's audio issues have also been a consistent problem on the Karma Grip gimbal as well. GoPro even went to the extent of enabling a audio 'filter' to remove parts of the audio file automatically to reduce the background motor noise while connected to their gimbal, but still even with the latest firmware the footage we reviewed of the Karma Grip this past week from our main contact at GoPro (even after he did some audio processing) still had a very bad muffled sound to it - almost like it is in a wind tunnel or echo chamber
The GoPro Hero5 Black the microphone positions and lack of control through the current settings menu is leaving many scrambling to find another audio recording solution. If you have a chance to try the Virb Ultra 30 I would highly recommend it as it does not pick up motor noise and actually has a decent battery life.
With the Hero5 Black you may want to consider trying an external microphone or lapel mic either using the external microphone adapter with a light weight USB-C extension cable or even even consider trying a separate external recorder. Rode actually has a smartphone app that works pretty good with a few different Lapel mics - we tend to prefer the external H1 zoom recorder.