I think you need to be part of the iTunes affiliate program to use the preview. The first sentence of the Legal section of the search API reads:
Affiliates may use certain advertising content that may be provided by Apple, including previews of songs and music videos, album art, and app icons
And on the Affiliate Program page:
Affiliate Tools: Many advanced tools are available to help tech savvy partners create links to the iTunes, App Store, and Mac App Store. These include the Search API, an enterprise affiliate feed that provides an enhanced feed for the entire store, and a Web iMix tool that you can use to create a dynamic playlist.
It remains unclear ... but I would not risk it, I join affiliates! In addition, you can make a profit if your application leads to a purchase. Apple Affiliate Program
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