I am thinking of creating a map using music. Is it OK to use music that is available on Fortnite (ex: PSY’s Gangnam Style, etc.)?
If it is not allowed to use music whose rights are owned by others, can I use my own original work without any problem?
If anyone knows whether or not they can be used or what precautions should be taken when using them, I would appreciate it if you could let me know.
https://www.fortnite.com/news/fortnite-island-creator-rules
umm, doesn’t say anything about music,
but there’s the Island Creator Rules 
Maybe this will help
Here’s Fortnite’s Supplemental Terms:
https://www.fortnite.com/unreal-editor-for-fortnite-supplemental-terms
Pretty much what it says:
*but there’s more if you click the link
“you must fully own that music OR you must have obtained the necessary licenses to upload”
Personally I would stick with Epic approved tracks or music you personally create, or music that you purchase for in game use as long as it comes with a very clear usage guidelines agreement.
Definitely stay away from any tracks that you find on YouTube or any non-Epic approved music. Music that is in FN that is created by other creators is definitely not permitted and the audio tools will probably flag that map right away. Just because it’s inside of FN doesn’t mean it’s ok especially now with all of the licensed songs they have in there. Gangnam Style? def not allowed.