Just wanted to surface this link to the old forum in case anyone is having the same problems that I was.
Getting the Tsunami Arduino library example running on a Due, and kept getting “‘Serial1’ was not declared in this scope”
To solve this I looked in Tsunami.h and replaced the following line:
#include <HardwareSerial.h>
With this line:
#include <Arduino.h>
Worked for me too. All up and running and making sounds now!