I've seen mixed answers about being able to hear the soundtrack. We went on the cruise May 2004, and we did have a receiver in the boat to hear the soundtrack.
As for is it better than being on Main Street, it all depends on what you are looking for. We saw Wishes several times on that trip. The first time from Main Street, then the Wishes cruise, we also saw it from our balcony at CR and another position in the park. I would say, don't miss it from Main Street. The Wishes cruise was so special another whole level, though. It was just the 7 of us with our captain, so it almost felt like a private showing. There was a tour of Bay Lake and the Seven Seas lagoon before, and it was just fun all being together doing something different.
Beth