Welcome to EP.
I have tried many different transistors but they all appear very slow?
Slow? What exactly have you tried (circuit diagram)?
I need to plug into the ear piece and have a LED flash to the metronome.
The audio output of the laptop carries a tone (click) for each tick of the metronome?
To make the LED flash visibly I recommend you use a monostable multivibrator to generate a pulse of a defined length. A simple circuit using a 555 chip is e.g.
shown here. A calculator for determining componnet values is included.
The output of the 555 can directly control an LED with a suitable current limiting resistor. Assuming a 9 V battery for power supply and a red LED, a 1 kΩ series resistor is good. Connect the resistor to the output of teh 555, then to the anode of the LED. Connect the cathode of teh LED to ground (0 V).
What's left is a suitable trigger circuit. An NPN transistor (almost any type will do) and a pull-up resistor should do the trick:
You may have to crank up the volume of the laptop's audio output for a reliable trigger.
