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