... I found this that may help you
To set the notification sound go to Home Screen>Menu>Settings>Sound & Display. Under Sound & Display scroll down to Notification Sound (Set your default notification sound). If you have downloaded other apps that can apply sound such as Zedge you have to choose which app to use to apply the sound, the default HTC Ringtone Picker or one of the other apps. Click on the app name to choose it and see a list of all available notification sounds. Click on one to mark it and hear it play. When you have the one you want click OK and you're done.