Some while ago I explained here how to do this on another topic inside this forum with use of Adobe Audition.
Search for Getting mp3 to be the same volume, or click this link below:
-1 db is what I use for broadcasting, but you can set it lower for use on a car mp3-radio player, say for example -6 db. You can not put all 900 files at the same time to convert, maximum on my system is 50 to 60 files at once.