MP3 Gain is the very best program for adjusting volume level of MP3's and best of all it's freeware. I make all my MP3's 92dB's. Most MP3's downloaded off the internet have major clipping (volume to high causing distortion), but some have volumes way to low.
MP3Gain does not just do peak normalization, as many normalizers do. Instead, it does a statistical analysis to determine how loud the file actually sounds to the human ear. Also, the changes MP3Gain makes are completely lossless. There is no quality lost in the change because the program adjusts the mp3 file directly, without decoding and re-encoding. You can change them up or down, over and over with no quality loss.
You'll want to use the Track Gain feature, I've had the program do thousands of songs at one time. It's never messed up anything. Let me know if you need any help with the settings.