Download a SMS Tone theme from Cydia. With SSH or iphonebrowswer or whatever you like to use go into /var/mobile/stash/themes and copy that to your desktop and use that as a template. Grab whatever SMS tone you want and replace the sound file (must be in .caf form).. change the theme name and copy it back to the phone. If you need help on converting sounds into the correct file format do a search on .mp3 to .caf. I use Switch to convert to .aiff and then from there you can just change the file extension to .caf and it works like a charm.