Don't look for a folder with the preinstalled system sounds -- that's a protected system directory that isn't visible to the user unless the phone is rooted.
/storage/emulated/0/ is essentiallly the "root" user directory (not the true system root, but the topmost directory the user can access). One of the directories there should be /Notifications. If you don't see one, then you can create it using the file manager app, then put the sound file in that directory and see if it now shows up as a selection.
I'm not sure what format the stock sounds are. They're probably mp3, but could also be ogg or wav files.