Do you remember at the end of Book 6, Mundungus took everything from
the Black House to sell it in order to scrape a living? Amongst those
things would have most likely been 2 horcruxes... that's right, 2! In
Book 5, JKR specifically wrote that while the trio explored in the
Black House, they found " heavy locket that none of them could open".
This was the locket that was swiped by RAB (Regulus Augustus Black)
from it's true resting place in the cave. The other horcrux was a small
music box that played a twinkling tune which "made them all curiously
weak before Ginny had the sense to shut the lid". Just like to find the
locket you had to drink the potion to weaken you, perhaps at the end of
the weakening song, the horcrux would be revealed but the weaker Witch
or Wizard would have been killed-off long before...
so we know about the ring and the diary. and the cup and locket. we
haven't heard much about anything from ravenclaw. maybe thats what the
7th book will tell us.