Working with biosignals is a very interesting field of work<p>As noted, the mains hum is a major source of havoc. It is usually filtered very strongly (and you usually have to do some analog filtering, otherwise you'll get less of the original signal that you want - this also adds issues like phase distortion, etc)<p>You also have to use very low-noise/high-impedance input amplifiers, so either get the expensive parts or do something like this: <a href="http://www.allaboutcircuits.com/textbook/semiconductors/chpt-8/building-a-differential-amplifier/" rel="nofollow">http://www.allaboutcircuits.com/textbook/semiconductors/chpt...</a>
I thought <a href="https://www.youtube.com/watch?v=iVXOjVZNfhc" rel="nofollow">https://www.youtube.com/watch?v=iVXOjVZNfhc</a> was fascinating regarding EMG signals too.<p>Theres also some really amazing research attempting to decode inner speech via EMG - <a href="http://ti.arc.nasa.gov/m/pub-archive/471h/0471%20(Jorgensen).pdf" rel="nofollow">http://ti.arc.nasa.gov/m/pub-archive/471h/0471%20(Jorgensen)...</a>
This isn't a "microphone" at all. Microphones by definition transduce the physical movement of sounds waves into a signal. This is just using the sound card to sample the electrical signals picked up by the conductive leads.