Was struggling with this for a while as I mostly play Old School Hip Hop and some of my tracks I ripped from CDs YEARS ago so some had low volume while others were too high, and playing with the gain during a live set can be tricky. That said, I had great success using MP3Gain. Just added entire folder, set the Target “normal” Volume and ran it. All sorted, all tracks on level volume.
Download: http://mp3gain.sourceforge.net
