Now if someone could do a step by step instruction how to stop Google Voice from sending 2 messages out. Sure hope that gets resolved soon.
1) You can either use a Ringtone/Notification app such as Ringdroid or Mabilo.
I prefer to not use an app and customize myself. It's very simple. Go to your SD card and create two folders.... one called "ringtones" (all lower case) and one called "notifications" (again lower case). Now add the custom ringtone/notification tone or song you want to use into the appropriate folder. For Handcent this would be the "notifications" folder.
2) Open Handcent, press menu button, select Settings, scroll to 'Notification settings', then select ringtone. The custom file you added to the notification folder will be available from the selection list.
...if you want to create custom ringtones for each contact just open a thread with that contact and click on the icon on the top right hand corner of the app (looks like a person with a gear wheel in front of them)