If you want professional alert tones or snippets of popular movies and songs without editing them yourself, you can buy them directly from Apple. Navigate to > Sounds & Haptics > Ringtone . Tap Tone Store to browse and purchase new sounds.

Tap the (plus) icon to create a new alarm or Edit an existing one. Tap Sound . Under the "Songs" section, tap Pick a song . Browse your library for a downloaded track and select it. Tap Back and then Save . Downloading Official Tones from the Apple Tone Store

The simplest way to "download" a custom sound is to use a song already in your music library. Note that songs must be downloaded to your device and cannot be streamed from services like Spotify or Pandora to work with the native Clock app. Open the app and tap the Alarm tab.