The best way would be to run SEPERATE wire for power and speakers. Amplified speakers for this application would not be the best thing to do. The type of speakers that I would use, would be 70 volt commercial paging horns rated for outdoor use, and a 70 volt mixer/amplifer such as a Bogen or University sound brand. The city I live in has a festival during the 4th of July, and do the same thing. The wire isn't flown though, it is just ran along the ground and up a pole to speakers. The problem with leaving low voltage wire up year around, is that wire rated for that, is very expensive. a simple 16awg or 14awg 1 pair wire would suffice.
Hope this helps....
