Sometimes it comes as a surprise as a people to people when I tell them that every speaker is also a microphone. This is true because in a speaker you send in electrical signals to change the way a speaker cone moves. This in turn produces various sounds.
With the same setup, if one provides a mechanical vibration to the diaphragm, this will generate audio signals corresponding to that mechanical vibration.
We have made a video demonstrating this using a headphone jack and a phone. In order to loop the sound from the mic to the speaker in the video, we use the following command on Linux:
pactl load-module module-loopback latency_msec=1
The hacker Diary 
What is that thing you have plugged in the USB port of your laptop? It will be great if you can provide details on how someone else can do what you have done. Looking forward to more details on your cool projects. Thanks.
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That thing that has been plugged is an external USB sound card..
As far as more details are concerned, we will be updating the post very shortly.. Thanks !Updated! Hope everything is clear ! Thanks!
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