The first thing you need to do install a package called AppTapp that you can download at http://iphone.nullriver.com/

Once you have this package, plug your iPhone into your mac, and after iTunes and iPhone started up, just quit those 2 applications. Start the AppTapp installation, and you're done. Once you rebooted your iPhone, click on the "Installer" on the main menu, and install BSD Subsystem, Installer, and OpenSSH

After that, use a FTP client that can do SFTP, like Transmit (http://www.panic.com/transmit). Once this is installed, go back on your iPhone (I suppose that you have it connected to your WiFi network), and get the IP address (Settings -> Wi-Fi -> Click on the blue arrow on the right of your network name, and get the IP address). Start your SFTP client using your iPhone IP address. Login is root password is dottie

The folder you want to go to is /System/Library/Audio/UISounds

Use a software like FISSION (http://www.rogueamoeba.com) and convert your wav/mp3 files to an AIF. Once this is done, on the iPhone, rename the file sms-received.caf into sms-received.caf.old and rename ReceivedMessage.caf into ReceivedMessage.caf.old. One file is for the SMS, and the other one is for the emails. When I changed my SMS file, it did play the one for the emails. So instead of going crazy and see why it is doing that, I just changed the sound for both files. So copy over your .AIF file, and rename it into sms-received.caf and ReceivedMessage.caf

Here is 2 files that I made: One from Spoke / STAR TREK: CAPTAIN, INCOMING MESSAGE, that I use for incoming SMS. And the famous AOL / YOU'VE GOT MAIL for new emails.. You can directly copy those files into the appropriate iPhone folder. REMEMBER TO BACKUP YOUR OLD FILES FIRST !

You're all done !