My mobile phone is Sony Ericsson X8. I want to set a short message ringtone. However, you can only select the system defaults. You can't choose your own downloaded songs or something,

Settings-sound and display-notification ringtone-there is a music library at the bottom, which is to find music from the mobile phone.

If this method does not work.

Insert your SD card into the computer, open it and find the ringtone folder. This is the default ringtone folder. Put the music you want to make ringtones in this folder, then insert the SD card back into the phone and enter the notification ringtone. You will find the ringtone you want in it.