I think the cross is pure luck. In my case, I think we were just randomly playing with friends in my home, and falling down the screen just randomly hitting with the sword we heard the sound! It was just a matter of patience to figure out the way to get it
What about the Doll? Do you get a hint for that? That & the Cross (indirectly) are the only items hidden behind a destructible wall not? Regarding the Cross, you also have to (accidentally) know to jump into/against the wall...
Yes there is a hint for the doll; what it does in the manual. I think also something in the game. And if you defeat the warping medusa head you hear a sound that you solved something. That is your cue to go investigate...
Quick question about shrines:
I read that there are 20 in total, but I counted 19. Where's the unaccounted for one?