My thought is that for two-way, you will need to toggle the VMUSIC2 CTS#, so instead of just grounding that pin, you'd attach it to a digital output pin and do something like
digitalWrite(CTS_pin, LOW);
Serial.print("V3A"); #play all files
Serial.print(13,BYTE);
digitalWrite(CTS_pin,HIGH); #allow data to be transmitted from the VMUSIC2 to the Arduino
while (Serial.available() > 0) {
incomingByte = Serial.read();
... # do something with it
}
Unfortunately I can't test this for a few days...too many projects going on at the same time! But I'll post results if it works.