I have a co-worker who just installed some speakers that are truly hidden speakers. You texture, prime, and paint right over them after install.
The company is onsia Onsia Concealed Sound Solutions
He seems to be happy with how they work. They do however transmit through both sides of the wall. He has them in the ceiling in basement theater system and the sound comes through the floor upstairs. Could be good for filling the house, if you're having a party, but not so good if you're watching the latest action thriller and the kids are supposed to be asleep in the next room.