The big advantage of those flexidiscs / soundsheets was that in addition to the low cost and ease of duplication, they could also be glued into the binding of a magazine, and did not require any special protection or packaging, due to their flexible nature -- unlike floppy discs or CDs, which must be packaged separately in a plastic bag along with the magazine, since there is no convenient way to stick those into the binding and make it through the magazine's mechanized printing/stacking process without getting ruined.