Audio masking techniques are used for covert listening devices, concealing intercepted audio from anyone trying to eavesdrop. One common method is subcarrier modulation, where an audio signal is added to a higher frequency subcarrier, then onto an RF carrier that sounds silent until further demodulation. This technique was used in bugs by intelligence services during the Cold War. Another technique is the rarely used Double Sideband with Suppressed Carrier, making it challenging to detect the bug. Pulse Position Modulation is another technique developed for the CIA in the 1960s. Discovering these bugs can be challenging, especially with techniques like Rejected Pulse.
https://www.cryptomuseum.com/covert/bugs/masking/