Using a free VST app you can use the mic in to control MIDI events. It can be used to control drum machines or simply trigger samples.
Step 1: Equipment and Tools
set of old headphones (only one side needs to work)
empty soda can
cassette tape case (or whatever case you like)
Step 2: Cut a Hole in the Cassette Case
I also cut a notch on the side of the case for the cord to fit out of.
Step 3: Cut the Pads
I cut a second section of mouse pad slightly smaller than the lid of the cassette case.
Step 4: Prep Ear-bud
You can use a pair of broken headphones as you only need one speaker. Cut away the broken one.
Step 5: Glue the Assembly Together
Glue the section of pop can to the under-side of the "trigger pad."
Fit the exposed ear-bud into the hole, and glue the "trigger pad" in place.
Make sure the metal of the pop can that is glued to the "trigger pad" is touching the metal of the ear-bud.
After this was all dry I used black tape to finish off the look and disguise the cassette case.
UNOPAD (get it?)
Step 6: Install Software
A plugin that triggers MIDI notes based on the sound level of the incoming audio stream in different frequency bands. It allows you to "detect" occurrences of percussive sounds in an audio stream and send out a MIDI event whenever that happens. Available as VST effect for MacOSX and Windows.
Virtual midi cable program ( MIDI Yoke or Maple Cable)
You can use Ableton, Cubase, Cakewalk, Fruity Loops, etc. to run the VST
a Stand Alone VST Host. I suggest SAVIHost
Set the Midi Out of SAVIHost to Midi Yoke 1. Then in your audio app, set Midi In to Midi Yoke 1.
In the Wave Devices settings of SAVIHost set the Input port to your mic input.
In KTDrum, make sure you set the Threshold and Gain of the Trigger 1 high enough to trigger events, but not to low enough for ambient noise to set off.
Have Fun and make Great Music!