Advanced Audio Coding
Advanced Audio Coding (AAC) is an audio coding standard for lossy digital audio compression. Designed to be the successor of the MP3 format, AAC generally achieves better sound quality than MP3 at similar bit rates. This is the native audio format for iOS devices.
When you import audio files directly into OnSong, they are added to your OnSong audio media library where you can link them to chord charts as backing tracks.