Thanks for sharing!
It could be done in Fortran. I have written a C teaching project to synthesize sounds and write them in a WAV format. It was inspired by:
https://gasstationwithoutpumps.wordpress.com/2011/10/08/making-wav-files-from-c-programs/
A WAV comprises a header with metadata then the soundtracks in PCM (Pulse-code modulation - Wikipedia). And you need a little endian procedure to write the 2 or 4 bytes data.
The Xenakis’ book is here:
https://books.google.fr/books?id=y6lL3I0vmMwC&printsec=frontcover&hl=fr&redir_esc=y#v=onepage&q&f=false
You can read some parts of the Fortran code around page 317(but I don’t know if it is the code of that music, we can preview only a limited number of pages).