So I played with it again for a while, and I ran into the issue of inconsistent volume levels of my MP3-collection: some mp3′s sound a lot louder than others.
After some ‘googling’, I found a fantastic open-source program to adjust mp3 files so that they have the same volume: MP3Gain. This little piece of software is super! The changes MP3Gain makes are completely lossless, because it adjusts the mp3 file directly, without decoding and re-encoding: the mp3 format stores the sound information in small chunks called “frames”. Each frame represents a fraction of a second of sound. In each frame there is a “global gain” field. This field is an 8-bit integer (so its value can be a whole number from 0 to 255). MP3Gain analyzes the song, and then adapts this value without touching the rest of the mp3 information.
Better yet, MP3Gain does not do a simple peak normalization but it analyzes mp3 files to determine how loud they sound to the human ear. It can then adjust the mp3 files so that they all have the same loudness. This is based on the Replay Gain algorithm (detailed information).
Probably best of all: MP3Gain is completely freeware. No registration, no time limits, no disabled features, no annoying pop-up messages, or anything like that.