At Sea Breeze Point, you would need to rent a sound system from Disney or bring a battery-operated one of your own. The $800ish price floating around is for a sound system at Epcot that includes a sound tech to run it (i.e. play the CDs or MP3 player you bring). The price of a sound system varies by locationit may not be the same at Sea Breeze Point. Also, you don't necessarily have to hire a sound techDisney can rent you the system and you can have one of your guests run it (most likely it would just have a cable where the guest plugs in a CD/MP3 player and controls everything from the device). That would be less than $800 because you aren't paying for a tech.
People have brought their own portable, battery-operated systems to SBP before. Again, you'll have to designate someone to run it, which you may not want to do. But it's an economical alternative.