You can follow below steps to add Custom Notification/Ringtone Sound to your Samsung Galaxy S6
1. Open the My Files application(The default file manager on Samsung devices).
2. Under Local Storage, open the Device Storage option
3. Browse to the audio file, long press on it, tap MORE in the upper right, and select the Copy option
4. On the new screen that appears, under Local Storage, open the Device Storage option
5. Under Device Storage, find the “Notifications” folder (or Ringtones folder if you are adding a ringtone), open it, and tap PASTE HERE
6. Open up your phone’s Settings
7. Under Device select Sounds and Notifications
8. Select Ringtones and Sounds
9. Tap either Ringtone or Default Notification Sound, depending on what you are changing
10. On the opened list, select your audio file to activate it.
You are done, enjoy!!