Adding audio files from within Airmix require the music to be synced to the Apple Music library. For Windows users and those who prefer not to have to sync with Apple Music, you can import music through the Apple Files app.

1. Open your Files App
2. Navigate to the audio file you want to import and tap it
3. Tap the Share button in the upper right corner.
4. In the list of apps, scroll over to find Airmix
5. Tap Airmix to load your file into the Airmix app