Many years ago I made an audio scrambler with an MC1496 balanced modulator/demodulator IC, a crystal oscillator, a counter and a couple of switched capacitor lowpass filter ICs. It was for a wireless FM microphone and made single sideband suppressed carrier signals. The high audio frequencies became low frequencies and the low audio frequencies became high frequencies and voice words were completely unintelligible. The descrambler used a very similar circuit and tits sound was perfect. It worked so well that my prototype was sold as the finished product.