Textfree Unlimited App - Sound Notification

I have the Textfree app, and also upgraded to iPhone 3.0 OS. This allows me to take advantage of notifications from Textfree. I have the sound notification working, but I want to know if there is a way to change the sound that Textfree uses.

Currently it is the same as the system sound for 'New Mail' found in Settings>General>Sounds>'New Mail,' but I want to change it to something less annoying, not necessarily a true ringtone, but something other than what it is currently.

Any help is appreciated!