Apple added a new function that will now allow you to set a separate tone for Voice Mail, New Mail, Sent Mail, Tweet, Calendar and Reminders. What they also have added is a Tone Store which you can buy new tones for .99. Here how to do it.
Go to the Settings app and select “Sounds”
Once in the sounds settings choose which alert you want to change
Then either select the tone you want (you can also use any of your current ring tones) or at the top of the screen select “Buy More Tones”. Then you can buy a tone and at theta time iOS will ask if you would like to set the new tone.