If you have a Pro subscription, you can add music to your beautiful mashes to make them more memorable.
iOS
To do so, follow the usual steps to add music, but tap on "My Music" instead of a 1SE track. The Music app will then pop up.
Please be aware that publishers use digital rights management (DRM) technology to protect copyrighted material. This technology prevents songs from being illegally used or distributed. You can read more about it here.
DRM means that you cannot use music you don't own, for example, music from your Library that you have downloaded using an Apple Music subscription. You'll need to purchase and download the song from the iTunes store to use it in your mashes—you can't use music stored on iCloud.
Android
Follow the usual steps to add music, but tap on "My Music" instead of a 1SE track. This will open the file explorer. Select the music (.mp3 file) that you want to use as background music.
Depending on your Android phone, some devices have a sound picker that automatically scans all your music files for easy selection.