Ditto! I also just registered to confirm that this is the way to stop the audio volume feature, if unticking the sound check box on Preferences does not work. It's funny, the method described above I figured out just minutes before discovering this thread!
To double-check, you can also right-click on the track, select "Get info", and the "Summary" tab with the file's description should NOT have an item that says "Volume". If it does, then, the audio volume feature is not fixed (if it is not, where it says "Volume" should read, for example, "-2.8dB". If you see this, then try unticking the sound check box in Preferences or the method described above).
Edited to add: Also, make sure that the Sound Check feature on your iPod is turned off. You can find it on the Settings menu.