I'm pretty certain you are going to be out of luck, sorry. My understanding is that ALL Soundpool purchases are "final", with no possibility of a refund.
However, depending upon how recently you actually purchased the Soundpool, it might be possible for you to request a cancellation. (The standard period is up to 14 days from the purchase date.) This page gives the full cancellation policy under which Magix operates.
@ross-carruthers You accepted the terms of service when buying the Soundpool. Refunding your purchase is pure goodwill and Magix doesn't need to do it. But they can to keep you as a customer.
There is very little point in making threats in the forum about what you are going to attempt to do through legal channels in an attempt to secure your refund. Magix' Terms and Conditions are quite explicit and clear. By default, you accepted those conditions when you made the purchase. Magix (the company) has done nothing wrong if they refuse you a refund if it is outside the 14 days; you admit you made what was, at the time, the "correct" purchase. Subsequently, you decided that you wished to "upgrade" that purchase. Just because Magix do not offer a direct route to achieve that upgrade without you making a whole new purchase and not being credited with a refund of the original purchase price is clearly implicit in those terms.
Unless your refund request was made within 14 days of your original purchase Paypal will not issue you a refund because Magix have not done anything "wrong". And I'm pretty certain the German trading authorities will take exactly the same view.
Because there is no point at all in continuing this discussion I am going to close this thread.