-normalize [ VOLUME ]
If used without VOLUME argument, analyze the audio stream and then normalize the volume of the audio with the "normalize" program. This is useful if the audio is too quiet or too loud, or you want to make volume consistent for a bunch of videos.
Similar to running normalize without any parameters. The default is -12dB average level with 0dB gain.
If the optional VOLUME argument is supplied, ffmpeg will be used instead to increase or decrease the volume. An argument of 256 is normal volume.
This option is useful for -slice, because there are sync issues if audio and video are encoded separately after a seek.