I had seen a few people mentioning their audio files not looping when using the Radio device; this is because, although it states files are played in a loop, it only refers to the already existing “Cue” files (such as those seen in the device’s sound options).
So it’s actually very simple and quick to do. When you import your audio file, convert it to a Cue by right clicking and selecting “Create Cue”, then double click on the Cue file. This will open up the sound cue. From here you simply have to create a “Looping” node and link your cue to it. Done.
Your audio will now loop indefinately until stopped.
https://dev.epicgames.com/community/learning/tutorials/oP7a/fortnite-looping-custom-audio