Remove this line
Serial1 fxSerial(rxPin, txPin); // Software Serial object
Change this
fxSerial.begin(9600); // initialize Serial communication for mp3 player
to
Serial1.begin(9600); // initialize Serial1 communication for mp3 player
Change this
fxPlayer.begin(fxSerial); // tells mp3 player to use fxSerial for communication
to
fxPlayer.begin(Serial1); // tells mp3 player to use Serial1 for communication