Cpup

Hi, I have a record label website and selling my music directly, but I want my customers to be able to choose between mp3 or wav. I've tried creating 2 identical products with different product codes, one visible (wav) and the other hidden (mp3). When I select the product and choose mp3 and do a test purchase, I get the (wav) product still? Is there a hook or an extension that can do this? The product options only seems to cater for tangeable items that require shipping. Thanks...