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Oh okay, well one thing you could utilise if you have a computer, maybe a laptop or whatever is using free software called audacity. Upload the file from your phone to computer and open it in audacity. It will only open the audio of the video file. Once it's opened click File>export>Save as wav file
Then you have a separate file for your audio. You can do some production on it. I use bitwig for audio but if you want something free try Ardour.
You can use EQ to get rid of the frequencies you don't need. Then use a limiter or 2 or 3 to raise the volume.
The reason I say 2 or 3.. is because sometimes it is better to increase the gain bit by bit, try 2db then add another limiter rather than boosting one all the way up.
Once your finished with editing your audio you need to sync it back to the video, You can use Kdenlive to do that. Its a free video editing software. Just remember to mute the old audio track in your timeline.

Awesome! I’m going to try that on this weeks open mic video for sure. Thanks for taking the time to tell me about all this @thesimpson!