There was a CD and/or a website that let you mix the artist's songs yourself.

I'm thinking the Sigur Ros website had a feature similar to what I'm talking about when () came out, but I cannot find anything there now. But, I'm pretty sure that there was also a CD that came with a program that let the listener custom mix the tracks.

What the hell am I talking about?

i think you might be referring to happy music for happy people by mogwail that came out last year or the year before.

i never did it myself, but i remember reading it in the liner notes.

Didn't the Beastie Boys do that also for their Greatest Hits album?