Grove - MP3 library also can be found in Seeed Wiki, or you can simply include MP3.h in project folder.
Seeeduino's digital pin 2 and 3 act as a software serial, it controls MP3 Grove by using the protol in KT403A datashell. After Grove MP3 and Touch Sensor are initialized, set Touch Sensor's sensitivity and Grove - MP3's volume.
The loop() method keep on checking if Touch Sensor is touched or not, if it is touched, play the corresponding mp3 file in current instrument folder. Current insreument can be changed by input instrument's index via hardware serial.