Your modulating signal is 250 Kbaud, which would mean that tryign to FSK it to frequencies lower than 250 kHz or so will not work.
Recording DMX-512 as it is on DMX-512 line to CD or audio tape using FSK modulation is a lost battle. There is not enough audio bandwidth on the tape or CD that you could fit in the 250 Kbaud data rate to it using simple FSK modulation. Getting the 250 Kbaud to fit to normal audio bandwidth would need at least far more complicated modulation (multi level modulation system that can fit several bits on one modulation symbol or multiple modulated carriers working in parallel). I don't think that this type of system would be practical.