Everyone has already done this but this is the what I did. Included is a sound sample from plugging the mike directly into my computer. I have not played in twenty years so it will be pretty bad technique, but you see it works.
Step 1: What's Needed.
1 - acoustical guitar.
1 guitar amp.
1 - female phono plug
1 - Good piezo buzzer
1 guitar phono plug cable to go from guitar to amp.
double stick pads
Drill and bit.
Step 2: Title This StepA Palce for the Phoneo Plug.
Drill a hole for the phone plug. size will depend on the threads.
Step 3: Soldering
Cut two wires long enough to go from the speak to the edge of the guitar with the hole is.
Solder wires from piezo speaker to phono plug
Cover connection with electrical tape,
Step 4: Putting It Together.
Make sure guitar has no strings. Best time to do this is when you need to change strings.
Put soldered units in guitar but do not let go.
Arrange the the phone plug so it fits through te hole you drilled.
Screw on nut for phono plug.
Gently lay down piezo speaker.
Put double stick layer on support near the bridge.
Place piezo speaker on double stick layers.
Set aside to let the stickum dry for a bit.
Note: Removed original picture for updated picture.
Step 5: Your Done!!
Plug in guitar cable.
Plug in amp.
Turn on volume so to make sure you do not have reverb.
Turn on power.
Turn up volume to desired level.
Add music to a single picture.
$ ffmpeg -i ss.png -i git.wav final.flv
Play the file/
$ vlc final.vlc