Wow! you're only in 7th grade and you're using Prezi? That's impressive!!
To have a continuous looping sound in your Prezi you're going to want to insert an .SWF sound file. Here's how...
1. Upload an SWF sound file like an image (Insert/File in the bubble menu in Edit Mode.
2. Use your zebra tool to move it around the way you want it. Remember the .SWF naturally doesn't have any image so the sound file will be invisible on your prezi but the sound will continually play.
Also for future use, here's the instructions for basic sound usage with prezis...
Good Luck with your project, I'm sure your teacher will love it!