In science fiction movies, computers are often seen communicating with one another (or with humans) using robotic voices.
Today, most computers don’t communicate with one another using real audio and instead choose to silently send bits and bytes of data to one another. Unfortunately, it seems like times have changed.
One recently discovered malware replicates itself using speakers and microphones on your PC. Audio information comes out of the speakers on one PC. The microphone on another PC overhears this audio information and the malware spreads itself over the air.
The researchers who initially discovered this malware, called badBIOS, couldn’t believe what they had found. After extensive testing, they discovered that the virus was indeed spreading itself over the air using audio signals between speakers and microphones.
This virus is frightening because it has the potential to affect systems that were previously closed off to the outside world. Some systems, for example, are not connected to the internet in order to protect their security. If a laptop or computer connected to the internet got close to one of these rigs, then this audio malware could jump to the isolated rig and cause some major problems.

Here’s how this virus was discovered:
-The signal was propagated using a software-defined modem based on the GNU Radio project
-Researchers were able to ping the malware signal back and forth between systems
-The data transfer speed is low at about 20 bits per second, but that’s still fast enough to transfer the malware to four PCs in about 18 seconds
-In most cases, people ignore the sounds coming out of their PC speakers as “background noise”. This malware just outputs a few weird clicks, clacks, and buzzes from your speaker, noises that could easily be interpreted as innocent interference.
-This malware currently isn’t widespread, so don’t get too paranoid about the sounds you hear coming out of your speakers just yet
Source: http://www.extremetech.com/computing/171949-new-type-of-audio-malware-transmits-through-speakers-and-microphones