1) 12mm Tactile Switch (http://adafru.it/1119)
2) 3.5mm Stereo Headphone Jack (http://adafru.it/1699)
3) Stereo 3.5mm Audio Cable (http://adafru.it/876)
4) Two 4-40 nuts & screws (~1/4in)
Using as thin a wire you can find, solder the switch to the tip and ring of the 1/8" jack. Bend the leads underneath so that they fit in place, and screw the two sides together. Tighten once around the syringe.