When you’re talking, things in the same room vibrate from the sound waves. Film the vibrating object — e.g., a bag of chips — run it through some software and Bingo! you can turn it back into speech.
Both the police and your neighborhood peeping tom will love this.
[Scientists reconstruct speech through soundproof glass by watching a bag of potato chips http://www.theverge.com/2014/8/4/5968243/mit-turns-recorded-vibrations-back-into-speech-and-music via DuckDuckGo for Android]