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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.

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I think what you're talking about is called volume normalization. – David Kohen Oct 23 '12 at 12:59
OK - changed the title and post. Would you also happen to have some clues as to how to achieve it? – Emanuel Berg Oct 23 '12 at 17:59

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You mean mplayer's output, e.g always 80%, right?

If so, use -volume option, e.g mplayer *.flv *.mp4 -volume 80

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It sets the mixer volume, it seems (so if you use mplayer with -ao alsa, you can see the change with alsamixer). But won't a loud file still be louder, as the mixer volume is the same for all files? (Correct me if I'm wrong.) – Emanuel Berg Oct 21 '12 at 17:11
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Here's a link on how to do it on VLC: videolan forum

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Close (+1) but not quite: This will change the volume constantly, driving you crazy. The volume setting must be the same for each song. If the volume of one file is set high, the volume of the player must compensate for this (reducing the volume), but only at the beginning of each song. Perhaps the way to go about this issue it: for an .mp4 or .flv (or just about any media file), how would you extract the default audio level? I suppose, for each file format, this data is put in a specific header field. – Emanuel Berg Oct 24 '12 at 21:03
It is not data in the files all the time, LAME adds a volume level to files it encodes (at some point only using a switch, at another it was by default) but it is not part of any standard that I know of... – David Kohen Oct 25 '12 at 15:25
Could you somehow determine what files are loud and what are not? If so, I could write a script to compensate, once for each file - I'd get sort of normalization, but on a per file level (and that's what I desire, otherwise the music gets screwed up - try it yourself). – Emanuel Berg Oct 25 '12 at 18:54
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There is a script that reencodes audio in video files according to the gain, keep in mind that this may lower audio quality: http://www.jwz.org/hacks/video-replaygain.pl

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