You may have a lot of mp3 files and each one of them in a different volume level due to different compression formats and rates. That means you'll listen to some of your songs higher than others. MP3Gain is a lite but useful program that will normalize all of them to an only one volume level so you won't have volume problems. It's really easy to use and the rocess is fast enough. Choose the songs you want to normalize and let Mp3Gain do its job.