CD covers are always designed for print, since they are usually mass produced for record labels. Also, the RGB mode is for the Web, as I said above, so naturally you wouldn't design something for print in RGB mode.
What's the difference?
CMYK identifies the four colors used in traditional printing presses, and stands for, respectively, cyan, magenta, yellow, and black.
RGB refers to the so-called scientific hues--the additive primary colors red, green, and blue--that, when mixed together in equal amounts, create white light. Television sets and computer monitors display their pixels based on values of red, green, and blue.
CMYK is supposed to reflect the way inks on a press will look like when you use them to print your pages. It is the very nature that practically all printing processes produce results that overall will look less saturated and somehow darker than what a monitor - which is 'painting' color in RGB - can produce.
And even if you're doing one CD cover, printing it in RGB mode it will look totally different then printing it in CMYK mode. Why? Because like I said above - RGB for Web, CMYK for print. If your going to print something, and you want it to look its best, then naturally you would create your file in CMYK mode.