I need to manage volume spikes in my sound system?

I have a small gym in an office building. I use a CD Player or an iPod connected to an amplifier to play music for some of my aerobics classes. Sometimes the volume on the music downloads will vary from one song to the next and the volume will spike up in the middle of class; this is very aggravating to my upstairs neighbor.

I was told to purchase an equalizer to help level out the sound, is that correct? will any sound equalizer do or do i need to look for something specific?