I found an app for custom ringtones but can't seem to use them for text, email, voicemail notifications. Really unimpressed with the Lumia 920 notification tone selection, but love my phone!
Don't believe the OS allows for modification of other tones other than ringtones (except by carriers and manufacturers), so I don't think there is an app that can fix that, at least until a later version of the SDK allows developers to modify this