Use iFile or something similar to look at the theme file. It is simply a question of putting your own sound in the correct folder structure and activating your theme in Winterboard.
...you can easily create your own Winterboard theme to have a custom unlock sound. Just download any of the existing ones and have a look at them to see how it's done. Search for "unlock sound" in Cydia. Themes can be found in var/stash/Themes.xxxxx/ on your device once they are installed (where xxxxx is unique to your device).