Look up Serial.print and Serial.println for an explanation and guidance on how to use them. Once you have the hardware serial port working add your mp3 player using SoftwareSerial. You had that working earlier as you could use the 'next track' command. Once both work in the same program read data from the SoftwareSerial port and write it to the Serial port.
I have an RFID card at work. The value I get from my scanner at home is not the same as the one reported at work, leading me to think that the software at work uses a proprietary algorithm to calculate the value.
If you want a different random number sequence each time it runs then you should add the line
(or similar) in the setup() section. This uses a random value read from an analog pin to seed the random number generator. If you want to repeat the same sequence for testing, seed it with a fixed value.