On this page you'll find the actual acoustic measurements. The following page should give you a better idea of what the cards actually sound like, though.
As expected, PowerColor's HD 7950 PCS is one of the quietest (albeit one of the hottest-running) cards. Interestingly, Asus' HD 7950 DirectCU II manages to outperform it in the gaming and full-load benchmarks, making it cooler, more quiet, and 20 MHz faster. HIS' HD 7950 IceQ Turbo finishes third in all three benchmarks. AMD's reference design comes in dead last, particularly when it comes to applying a load.
With the exception of the reference model, all of these cards deliver respectable acoustic performance.