Very easy fix: If you removed the charger compartment, that is the cause, just put it back. If you put the camera into the holder that blocked the microphones of the camera, that’s the cause, just drill big holes showing the microphones. Lastly check the camera holes, make sure no dirt or debris in there especially after water exposure. If none of those three fixes your audio muffled distorted issue then it’s probably the microphone itself or your camera speaker play back is bad, just play the clip on another device to see if the audio is any better.
Below is the fix mentioned by one of the gopro users. https://community.gopro.com/t5/Cameras/muffled-audio-Hero-5/td-p/20149
11-27-2016 09:01 AM
Re: muffled audio Hero 5
Hi, I have made an experiment with this issue, I have found that the audio is affected by the charging port door. if you remove the charging port door to use in the open case for ease of conecting the USB cable you will find the audio is very very muffled and if you
replace the door again the audio is fine ! I dont think anything can be done about this, I would say if you wand good audio then make sure you have the USB/HDMI door on and closed.