hard to answer your question without knowing OS and your skill of manipulating the phone and PC.
I was also puzzling the same problem as you had.
on my OS3.6.1A ringtone, cut into 20 seconds music as mp3 files located in “ringtones” folder under root directory are recognized as “custom ringtone” in phone ringtone menu.
on the other hand, OS 220.127.116.11A, files in “ringtones” folder are not specially recognized as custom ringtone but local music shows all your locally stored music files wherever they are stored.
oh i have more than 1,000 mp3 files. :3
then rename your ringtone files starting from “00” or else so that they comes to the top of the list.