Security/Espionage Idea: High pitched (invisible sound) code transmission/ Also, the Game of Duo

Security/Espionage Idea: High pitched (invisible sound) code transmission

What if you were to transmit/broadcast a secret coded message  at a very high pitch (frequency), like the pitch of a dog whistle or higher, that human beings couldn’t hear except someone (the person you want to receive the message) wearing special earphones designed to hear high pitched sounds?

The game of Duo –

1. 4 players have, between them one normal six sided dice and 2 six sided dices called Duo dices.
2. What are Duo dices? 1 of the Duo dices (a green dice) has zero on 2 sides; One on 2 sides and Two on 2 sides; the other Duo dice (an orange dice) has zero on two sides; Three on 2 sides and Four on 2 sides.
3. The 4 players first of all take turns rolling the ordinary six sided dice and, when they have each had a turn at rolling that dice then they roll the Two Duo dices.
4. Whatever 2 numbers are rolled with the Duo dices indicate which two players must pair up into a team of two (If, for example, they roll a 1 with the green dice and a 3 with the orange dice, then player 1 and player 3 pair up and player 2 and player 4 form an opposing team)
5. The two members of each team then add their dice rolls together and the team with the highest number wins.
6. The player that wins the game is the player that has been, after several rounds, in the most winning teams.
7. I don’t understand why there is zero on the Duo dices? – If you roll zero with one of the Duo dices, that means that the two numbers on the other Dice have to team up. (Understand, Roll a Zero with the orange dice and that means that players 1 and 2, the two numbers on the green dice pair up)