The Windows Phone 8.0 Update 3 brings the ability to assign custom ringtones to text messages, and you can assign a unique text message ringtone to individual contacts
After you install this update, you’ll know who’s texting you without even looking at your phone! You can also use custom ringtones for more things— instant messages, emails, voicemail, and reminders.
Here’s a few screenshot that shows how you assign a global custom tone for text messages and another screen that shows the field you’d use to assign a unique tone to a contact.
After you get the update, you’ll assign a global custom ringtone for incoming text messages by going to Settings > ringtones+sounds > manage app sounds > tap the “Messaging” option. You can also disable the notification that appears at the top of the home screen whenever the phone receives a new message:
You’ll assign unique ringtone for messaging to your contacts the same way you assign a unique ringtone to contacts for incoming phone calls…by selecting and editing your contacts via the People Hub: