To turn the audio volume up or down, you may use FFmpeg's Audio Filter named ​volume, like in the following example. If we want our volume to be half of the input volume:

ffmpeg -i input.wav -af 'volume=0.5' output.wav

In older versions of FFmpeg, the option "-af" wasn't implemented, so we had to use this:

ffmpeg -f lavfi -i "amovie=input.wav,volume=0.5" output.wav

More info here: ​http://ffmpeg.org/trac/ffmpeg/ticket/720