They take place before RtC, perhaps years before, they themselves haven't decided. They want to leave the exact time open. That's something I don't care for with the DW CDs.... the Virgin books had a definite order, in fact, the older Doctor books would say that the book takes place between two particular TV stories. Not only don't the CDs do that, but they just don't care about maintaining continuity with the books. They invent their own companions and it's hard to reconcile them with the books.