Quick reply...
When you choose a ringtone, you are offered a selection from "/home/user/MyDocs/.sounds/Ringtones". When you click button "More", you can browse the "/home/user/MyDocs" part of the device to select any audio file. Except, you cannot see a hidden directory - folder which has name starting with a dot, is considered hidden. And "/home/user/MyDocs/.sounds" is listed as "Nokia N900/Audio clips", with "/home/user/MyDocs" being named "Nokia N900".
I don't know where the "japan child / female song" is stored on your device. Do you remember the name it was listed under? Because, if you don't remember a keyword, you might have to listen to each and every audio file stored on your N900 until you find the one which you used to have as a ringtone.
Best wishes.
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