Are they pressed CDs or CD-Rs? CD-Rs in particular (but pressed discs as well) suffer from all kinds of problems that aren't always detectable by the eye. Two biggies are:
1. Initial burn quality (and burn speed). Drives throw out all kinds of errors. There is error correction built in, but it doesn't always work very well. Also... the faster you burn, the more errors they throw. Some drives are also better than others at burning, more compatible with different media types, and respond better (or worse) to different burn speeds. If you're curious, you can use Nero's CD/DVD Speed Utility to test your burner (the tab called Disc Quality).
2. Light exposure is the mortal enemy of CD-R discs. The dye layer is what it used to record data onto the disc (the laser "burns" a series of pits in the die, allowing the light from the reading laser to bounce off of the reflective layer and back to the lens of the drive). Sunlight and artificial light will degrade this dye layer too, just like the laser cuts through it. Of course, it does so in a mostly uniform pattern, but not perfectly so (reflections and whatnot), which introduces some imperfections, and the overall degradation increases the signal to noise ratio on the disc (and makes reading more difficult).
I would probably just attempt to re-download them. Etree.org has all kinds of live shows, from all kinds of bands. As does the Internet Archive and Lossless Legs, among other places.