[SkipJack]

I had a similar problem when i first got my phone. It would revert back to the default sound when I didn't place the sounds in the right place. The correct spot for messaging sounds (texts, emails, voice) are in the folder My Device>Application Data>Sounds. Then go to Start>Settings>Sounds & Notifications>Manage and make sure the sound is added in your tones. This should solve the problem if it is in fact this same issue.