Ah, good question. We were told that with all outdoor weddings, they recommend bringing your own portable CD player with batteries. They said they can't guarantee that there would be a place to plug in, or that the electricity would be going to all outlets.
Anywho, I was a good little Disney birde and brought our portable CD player. Unfortunately, at some point, with all the jostling, the CD player was turned on all night, perhaps longer. The batteries were drained by the time we went to go turn it on! I just ran over to the violinist who, alas, did not know You've Got A Friend In Me. She suggested Can You Feel the Love Tonight, so we danced to that. No big deal.
Our coordinator Juliana surprised us by (and don't quote me on this) STEALING a CD player from some unsuspecting resort worker. She popped the CD in, let us have our dance, then literally ran it back to wherever she got it. Yes, there was a plug at the back of the gazebo, and yes, it happened to have power at that time.
In conclusion, if possible, bring your own. But don't leave the batteries inside!
P.S. I was surprised by how great the violinist was. I assumed that I wouldn't be able to hear her, but she sounded great.