If you have the 2 CD release of As/Is you're still missing 26 (if I remember correct) songs out of the 40 total in the 4 iTunes releases. If you want those, I'd recommend buying the 4 iTunes releases (they're only $7.92 each if I remember correct) and then burning them to CDs.

If you're worried about the copy protection Apple puts on the files, you can make normal unprotected .m4a files out of them by using QTFairUse, a free program (Google for it).