The Poly has 3 different beaches, and the view of Wishes is fantastic from all of them. The one that is in the middle, closest to the featured volcano pool, is loudest and the most crowded, so if you want a more private setting, go to one of the others. The beach in front of Tahiti (toward the CR) probably has a slightly more head-on view than the beach in front of Aotearoa (toward the GF).
If hearing the sound track is important to you, you absolutely must be near a speaker from which you can hear the music. I agree with PP that you should arrive early to stake out a spot. Find a place where you can hear the Polynesian theme music playing in the background. If you can see the castle and hear the music, you're in a perfect spot.
While you're there, enjoy the Electrical Water Pageant at 9PM. It's short and sweet, but a classic!
We absolutely adore the sights and sounds from the beach at the Poly! They're even better with a tropical beverage from either the Barefoot Bar (near pool) or Tambu Lounge (2nd floor of GCH).
Aloha!