The program writes directly to the hardware of the CPC.
You can use the port with the adress address &EFxx.
Because the CPC has only a 7bit printer port (we need 8bit for the Digiblaster) we use the STROBE signal, which is inverted by the hardware, so you have to invert your sound value.
Now we want to output the value &C5:
LD a,&C5 ;A=&C5
LD B,&EF ;load the port adress address
XOR &80 ;invert the 7th bit
OUT (C),A ;out to the printer port