Online voice communications are widely used nowadays. To protect speech from leakage, people tend to initiate the talk in sound-isolated environments. In this paper, we reveal a novel attack that recovers high-quality speech from outside soundproof zones. The rationale of the attack is to leverage sound-sensitive characteristics of piezoelectric materials, i.e., a piezo film that can change the phase of reflected mmWaves when placed in a sound field. If the attacker transmits mmWaves and analyzes reflected signals from the piezo film, the speech information can be compromised. More importantly, the piezo film is paper-like and works without a power supply. We propose a new speech recovery methodology to transform sound waves into wireless signals and build an end-to-end eavesdropping system working as a through-wall “microphone” to recover high-quality speech stealthily. To combat signal attenuation and improve speech quality, we develop a speech-enhancement scheme based on generative adversarial networks and propose to use multi-antenna information for intelligible speech reconstruction. We conduct extensive experiments to evaluate the system. The results indicate that the system achieves over 98% accuracy for digit recognition and works well over 5m away through the wall. We also test the system under complex scenarios and give countermeasures.