Consistent volume on set of files (volume normalization)

If I have, say, 20 music videos (.flv and .mp4), is there a way to play them in mplayer (mplayer2) without having to change the volume for each video, instead specifying a desired general volume, and having mplayer adjusting automatically whenever necessary? **Edit**: Possibly this is called "volume normalization". By the way, I'll accept any solution for this, or even a description what is required. I use mplayer at the moment, but, if you solve this with, say, a microphone, a script, and vlc, that will be great. **Edit 2**: This is close to, but not quite "volume normalization": check out my comment to David Kohen's answer below.