If you are wanting to extract the sound no matter how poor it may be you will need to import the Video File into an editing program so that you can see the clip data. Sound is encoded as part of the video and can be extracted with an editing program and most likely that would be on your computer not your phone using an app.
Lol on my first video I learned the hard way the camera is no good for audio. It sounds like a jet is running in the background. I tried pulling the camera apart ando disconnecting the fan and it helped a lot but then the heats ink overheated in a minute. So the fan has to stay on. The next plan isomething to drill the camera case and pull the microphones out. They are in the inner upper frontal side of the camera. I even thought of cutting the wires and extending it onto the front tip of the steady grip. The dji osmo has a separate mic on the outside.... that's where I got the idea.
This mic issue is a real shame because the camera is amazing, consider recoding audio on a separate device then join the video from the steady grip with the audio from a audio recorder on Adobe premiere
The solution is to use separate audio and sync in post. The entire point of the "clapper" you see in production is to
give a single reference point for image and audio synchronization.