10
down vote
	

It isn't very well documented, but FFmpeg has a -vol switch which will allow you to increase volume output.

Example:
ffmpeg -i movie/endersgame.mkv -vol 512 -vcodec copy endersgame-newlevel.mkv

Infos:
https://wiki.archlinux.org/index.php/FFmpeg#Volume_gain
-vol 256  = 100%
-vol 512  = 200%
-vol 768  = 300%
-vol 1024 = 400%
-vol 2048 = 800%