On src\D_Main.c. Around the 860th line, you can see the files that are automatically added, so go to the bottom of that list, and under soar.dta, add your file. Then, under line 1236, make a new line, adding your wad's MD5 code. Make sure to set the new order of files right. It's probably gonna be 11, so scroll down and change music.dta from 11 to 12. And if you don't know what your wad's MD5 code is, just put anything there. Once you compile the exe, you'll get an error screen, with the MD5 that the wad currently has.