My phone stopped detecting custom notification sounds (but custom ringtone sounds still work??)
Any sound files stored in a folder named

Notifications


will be found by the system and displayed as an option for a Notification sound

Any sound files stored in a folder named

Ringtones


will be found by the system and displayed as an option for a Ringtone sound

Note that it doesn't matter where in the system your Notifications or Ringtones folders are stored, Android will still find them. Notifications in /media/notifications will be found, and so will sound files in /Music/Notifications. You can also have more than one Notifications or Ringtones folders.