I needed for a reliable, fast and non-destructive way to increase gain on a video’s audio track. I found it in using the ffmpeg tool, which is a swiss army knife for dealing with video and audio files. It’s conveniently cross-platform and works wonders.
To increase the volume of the first audio track for 10dB use:
ffmpeg -i inputfile -vcodec copy -af “volume=10dB” outputfile
To decrease the volume of the first audio track for 3dB use:
ffmpeg -i inputfile -vcodec copy -af “volume=-3dB” outputfile