I made some simple audio scramblers and descramblers using a balanced modulator IC MC1496 for a wireless boardroom tele-conferencing system. It was suppressed carrier, single sideband and it inverted frequencies. High frequencies were low and low frequencies were high. The output sounded weird and completely unintelligible. The descrambler made it sound perfect. The datasheet is full of example circuits of multipliers:
http://www.datasheetarchive.com/semiconductors/download.php?Datasheet=1323817