You would create the file and add a line for your custom music, following the pattern mentioned above.
For example, if you created Kelethin.mp3 and, if Kelethin's coords were something like 50,500,100 (I'm not sure what they actually are and can't check at the moment), and you wanted the music to be heard within a radius of, say, 1000 units, you'd put the following line into gfaydark.emt:
Code:
2,Kelethin.mp3,0,0,1.00,0,0,1,50,500,100,50,1000,0.00,0.00,0,0,0,1.00,1