Why I have been doing this to the movies I have:

   ffmpeg-normalize -v --progress -c:a aac -b:a 768k -ar 48000 -t -14 *mkv

To normalize the audio levels so that they don't get too loud or too soft. Though I wouldn't use it if you have less than 64 kbit/second/track.

https://github.com/slhck/ffmpeg-normalize