iTunes won't play/burn DRM-free tracks I've altered

Firstly, you don't need to use Nero to alter the volume of the tracks. Volume adjustment is available in iTunes itself for individual tracks or whole albums, by right clicking on a track/selection of tracks and 'Get Info', then look for the volume slider which will alter the volume not only on iTunes, but also on your iPod. iTunes will try and alter the volume of your tracks to match automatically although it isn't 100% effective- just tick 'Soundcheck' under Playback in your iTunes preferences menu. The good news is what ever volume adjustment you make in iTunes will carry over to your iPod.
As for the tracks you have created in Nero, perhaps you have saved them as a format iTunes isn't compatible with.

I just tried editing one of my iTunes Plus tracks with Fission, and iTunes was able to play it with no problem, so it's most likely got something to do with whatever Nero did to that file. Please tell me that you didn't save over your original file when you altered it in Nero.

1.25 GHz iMac G4 (Home) Dual 1 GHz G4 (Work) Mac OS X (10.4) 2G 20GB iPod (old school, yo.)

I don't understand- why should you waste cd's when altering tracks volumes in iTunes? Alter the volume, then listen to them on your computer or iPod before burning to cd.