I did log a feature request for this in mantis a long time ago, but it got lost. Its actually quite a complex thing to add - you would need to add functionality to scan the audio track of all your AV media files, and calculate a file-wide record level average, then write a scaling factor into an attribute that would get stored in the db and id3 tag/file. Then xine would need to be modified to read this at the start of playback and modify the current volume level based on this... but it couldn't be done transparently because you can't pre-scale audio like that when using digital audio connections, you have to actually change the amp volume. So you would need to implement the concepts of "current volume" and a temporary "playback volume" so that xine would change the amp's volume back to "current volume" at the end of playback.
Alternatively, you would need to find a 3rd party tool actually to permanently rewrite the audio tracks of all your media files based on the same calculation, so that LMCE doesn't have to do anything... this obviously could be dangerous as you can never be sure how well it will do this, or good well the audio sync will be afterwards... good idea to back up first!