boolean sender(int reading, int pin, int note, char msg[], boolean press, int threshold, int lowerthreshold ) { // This function is what does all the work
if(reading <= threshold) { // If the user stomps harder than a certain threshold...
if(!press) { // and if the sensor is not already in 'pressed' mode...
reading = reading/8 - 1; // convert the FSR reading to a MIDI-friendly value
midimsg(144, note, 128); // Send a MIDI message
Serial.println(msg);
What is that Serial.println() statement doing there? Is your MIDI device expecting a carriage return and line feed? Why are you passing msg to the function when it is always and empty string?
Why are you passing pin to the function when it does not use it?