The EZkeys Sound Expansion is available for all users who already own a product in the EZkeys Line.
The Sound Expansion contains no software but only sounds and therefore comes at a lower price. After your purchase, you can freely choose which sound library that you want to purchase.
If you do not own a product with the EZkeys software, you can not buy the Sound Expansion product since you do not have any software to run the sounds through. You would then need to acquire a full EZkeys product.