I have a Nexus 5X with Android 6.0.1 and latest security patches (June 2016). I try to use my own soundfiles for alarms and ringtones. I tried a lot of suggestions, but I can't get it to work. When I want to select my sounds, only the default sounds appear in the list.
First I tried to add them to the folders (/Alarms and /Ringtones) with the built-in file explorer. I could copy the files, but only the alarms were selectable, not the files in /Ringtones. So at that time alarms were fine, ringtones were not.
Here someone suggested that it could make a difference if the files are copied from a PC via USB cable. So I tried that, but it did not make a difference.
Someone suggested that the files might only be selectable after a reboot. So I rebooted and now it is worse than before: I cannot select my own alarms and ringtones. For both settings, only the default sounds appear.
I don't want to try the recommended "ES File Explorer" or the "Zedge ringtone & wallpaper" app. This problem must be solvable without any additional software. Does anyone have an idea how to find out why this is not working?
By the way: I tried sound files in different formats (mp3, flac, ogg). All files can be played on the phone. They worked perfectly as ringtones and alarms on my previous phone. The filenames do not contain any special characters.