NO NEED TO JAILBREAK
Copy the files found in the .rar file using i-FunBox to the following folder.
Go over to Raw File System:
The audio files once copied over will be named incoming.aif.caf
you will have to rename it and delete the ".caf" part for it to replace it and work. The same thing goes for the send_message.aif.caf.
After that open up WhatsApp and go to settings -> chat settings -> Chat Wallpaper and choose the first one in the wallpaper gallery.
That is it you will have to close the App at least once for the theme to load!
Enjoy the new iOS 5 iMessage skin and sounds!