If you're encoding audio to MP3 you could try lame's command line settings:
Scales input (every channel, only left channel or only right
channel) by n. This just multiplies the PCM data (after it has
been converted to floating point) by n.
n > 1: increase volume
n = 1: no effect
n < 1: reduce volume
Just your basic anime otaku some might even call nijikon.