Hi Holly!
The Christmas season is a magical season at Walt Disney World. Singing Christmas carols is a beautiful tradition and can appreciate your interest in that.
One of my favorite holiday happenings is the beautiful
Candlelight Processional. During this story of Christmas done with narration, choirs, and orchestra there will be a time when you are asked to join in during one of the carols. Of course I always have a hard time not singing quietly throughout the entire production.
Many of the resorts will have choirs or carolers come into their lobbies to sing and you are certainly encouraged to join in if you would like. You would have to check with your resort when you arrive to see if they have anything on their schedule during your stay.
Another opportunity might happen at
Disney Springs. There is a stage there where many groups come in and perform during the holidays. This certainly could include some caroling but again the entertainment schedule is not published at this time.
You will find many impromptu entertainment opportunities during the holidays in all of the four parks and resorts. One year I was with a group of panelists and we just started singing on a bus to the Magic Kingdom and had the entire bus singing away during the drive. It's things like this that make Walt Disney World so special and magical!