How to set default alarm ringtone in Android 6.0?
Answer is - use  "ES Explorer"

Very strange, but "System settings" have only Ringtone and Notification, and no Alarm-clock sound. I think it is a bug. Maybe that setting should be in Clock app, but it also no.

Steps:


Play any music file you would like to set as default ringtone for the alarm in ES Media Player
Click three dot button [:] in top right corner
Select Set ringtone
Now set ringtone dialogue will open. Choose Set alarm
Now open Clock app. You should not see the warning