I don't know this device but I've just had a quick look.
When you load the librarary, you should get an example sketch. It is this.
You can 'reverse engineer' it to get examples of the commands you require. For example:
To play track 10, would it be (lifted out of the example):
uint8_t n = 10 ; // track 10
Serial.print("\nPlaying track #"); Serial.println(n);
if (! sfx.playTrack((uint8_t)n) ) {
Serial.println("Failed to play track?");
}
So start by hacking a copy of the example sketch.