Post the code that is running on YOUR Arduino.
I basically want to connect a drum trigger to the Arduino
Have you done this? What is connected, where?
and when an input is given to it
Inputs aren't given. They are read. Are you reading anything from the drum trigger?
have that communicate over serial to the mp3 trigger board to play a sample on the SD card.
What is "that"? What are you sending to the serial port? Is the mp3 trigger board (link?) connected to the serial port?