Hello!
I have started a very complicated project where I am trying to build an Arduino Electric Guitar.
This guitar is going to have buttons for the frets and the plucking area instead of strings.
I've tried to put together a test project modifying this project:
I bought the Micro SD Card Reader and the Card itself with the adapter for the computer and wired the reader into an Arduino Mega.
So here's my first question:
How can I program the Mega to play a sound from the Micro SD Card when a push button is pressed?
Because the other project programs the Arduino to play a sound when a keyboard key is pressed.
So the second part of my project is that I have a speaker set with a 3.5mm jack that usually plugs into a computer. I bought a female 3.5mm input that has 3 pins. Two on the bottom and one on the side.
That leads to my second question:
How do I wire that 3.5mm input to my Arduino so that I can play the sounds from my Micro SD Card?
And finally my third question:
How should I encode my sound files so that they can be played through my Arduino?
I already have Audacity, a software that edits and encodes audio.
If you could help me out with this project test that would be great! 
--- DoubleK
Take a look at one of these
SD card socket is on the bottom, picture & more info here
Does it come pre-built? Becuase I can't solder very well.
You may have to solder headers on it looks like.
How can I program the Mega to play a sound from the Micro SD Card when a push button is pressed?
Because the other project programs the Arduino to play a sound when a keyboard key is pressed.
What kind of sound do you want? The Arduino can make basic tones (with the tone() function) but it's limited because it doesn't have a DAC. I think you can play different tones on different I/O pins so you can make chords, but you'd need to build a simple analog mixer circuit to combine the tones.
With an audio shield you can play an MP3 or other audio file from an SDcard, but one audio shield can only play one file at a time so you wouldn't be able to play chords on your digital guitar. (The audio shield has a DAC an a sample-rate clock, and sometimes an MP3 decoder, so the shield is doing all of the "work" and the Arduino is just controlling it.)
@DVDdoug I have a bunch of MP3 electric guitar note sounds and I want Arduino to play them
Something like this :- http://www.youtube.com/watch?v=ehy9xgl4YCs
It uses a MIDI sound generator to make the sounds.
But
Does it come pre-built? Becuase I can't solder very well.
Doesn't bode well for making your own project.