If you have volume issues between different mp3 files you can also put them through a free program called MP3 Gain
Just enter the files in the program and adjust the gain and it does a batch adjustment to all the files.
Quick and easy.
yeah normalizing tracks is a good idea, if you're unsure how to use mp3gain, is a good tutorial
another tip, if using xschedule, set up some buttons with say 50% and 100% volume. Then you can have volume down when no ones there, turn it up when people arrive, drop it back down when they leave (helps in reducing annoying the neighbours with too much noise )
in xschedule there's also a volume override, where you can reduce volume of particular songs.