I've never seen any sites do this before, but SideWinder Computer Inc. has made a sound recording for each of the fans they sell (which appears to be most of the top ones on the market). As I'm looking to purchase a new heatsink/fan, being able to hear how loud the Delta truly is, was a large plus. But, check it out. If you've seen any other more comprehensive sites, feel free to post them.
http://www.sidewindercomputers.com/tecar.html
http://www.sidewindercomputers.com/tecar.html