I shot a .mp4 video from my cellphone. Turns out that the volume is very low. How can I increase it?
You can use
ffmpeg -i file_with_quiet_sound.mp4 -af volume=150 -vcodec copy newfile.mp4
It will increase volume of your file at 150 times! Don't worry, big values can do funny effects, I like it. But I think, you will get many noises and not will love result.
If you want really make sound better, convert your video to
.wav file with ffmpeg:
ffmpeg -i file_with_quiet_sound.mp4 sound.wav
And then open it in any audio editor. I can suggest Audacity, this is free software with good features. I like old version 2.0.2 where exists old version of "Noise remove" filters which can produce magic with your sound (if you can do it right). If you don't like Audacity, you can to use any other audio editor. When editing of sound is done, you can join video and audio:
ffmpeg -i file_with_quiet_sound.mp4 -i fixed_sound.wav -map 0:0 -map 1:0 -vcodec copy newfile.mp4
Pretty simple. All software is free and available on every platform.