Get all the music that was needed to play via-CD in Quake
The music on the Quake CD has “pre-emphasis”. OK… what does that mean?
The hydrogenaudio wiki has a summary of pre-emphasis[wiki.hydrogenaudio.org] that is worth a look if you want details. The upshot though is this: to make the playback sound correct, ripped files need to be processed with a specific equalization curve to get rid of the pre-emphasis. This matches what your CD player did when playing the physical CD.
Ripping a CD with iTunes will handle such “de-emphasis” automatically. The hydrogenaudio wiki page has some tips about other ripping methods, and Google will also turn up good discussion about this.
The difference between Quake tracks that have been properly de-emphasized and those that haven’t is fairly subtle to hear. But a spectral analysis (showing the volume at different frequencies) can make what’s going on a little clearer. Here’s an example comparing the first Quake track not de-emphasized (top) vs. correctly de-emphasized (bottom):
Without de-emphasis, the higher frequencies will be too loud.
If you are using soundtrack files that you downloaded from somewhere else, and you don’t know whether or not they have been correctly de-emphasized… well, they probably haven’t been, but you might not want to lose sleep over it. Just something to keep in mind. If you want to test it, you can get the free Spek[spek.cc] tool, use it to open up your track02.ogg or track02.mp3 file, and compare the image you get to the images above.