Download Rings Extended from the Play store and use it to assign ring tones. Set the tones normally and you'll get the option to use Rings Extended -- that has seemed to work for me in the past with other devices experiencing the same issue.
I had an HTC evo 3d with the same issue. Kept getting replacements with the same issue. So I got my phone swapped for an epic touch and still had the same issue. I replaced my memory card and it was fixed. All my ringers were on my memory card. After all those exchanges for different phones it was just a bad memory card.
My Epic develops amnesia occasionally. I assign a ringtone to each family member and have one default for all others.
Why in the world does it change them?? I've had to reinstate them probably half a dozen times since December. I'm not pulling the battery or resetting to factory, so I just don't get it.
That's weird, do you have a ringtone assigned to a particular group of contacts that may overlap your family members? If you do, you have to set that group to default ringtone before it would work correctly.