Depends on a couple of things, I think the most important at this point will be the intentions of the player who threw the cards down and where the cards are physically when his intentions are made known,
If his cards don't hit the muck, and he declares to the dealer to flip them over for him they are still live and playable.
If his cards hit the muck, if the dealer picks them up and slides them to the muck before his intentions are made clear they are out of play. Edit some more; Anytime the cards hit the muck they are out of play no matter what the "intent". Intent may be clear but if the dealers shove them into the muck they are out of play. The devils in the details.
Some floormen may differ and put more emphasis on "intent" others on the physical location of the cards, then intent, but I think above is the most common guideline.
Edit added; If the cards don't hit another players cards, if they don't hit the muck they are playable if turned over by anyone ... they are playable.