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So I had a look, in the SLADE map editor, go to View, Windows and Script Editor. Or press Ctrl+5. This is where you enter ACS scripts. Don't get it confused with the Script Manager which seems to be for SLADE-specific scripts! Anyway if you do this in the Osiris maps you'll see how I did stuff. From the regular SLADE interface you can also just click into one of the wads in the pk3 and click on SCRIPTS to also have a look. You'll see a similar bit of code near the top, within "script 100 OPEN", in each map, which does stuff related to the intro texts and then sets the music when the map loads. 

So on your own maps, I expect you'll just get an empty document when opening the Script Editor (unless you've done some other ACS stuff). So basically for now I'd recommend putting the following bit of code in here:

#include "zcommon.acs"
#import "acs/osiris.acs"
script 100 OPEN
{
    SetMusicVolume(0.0);
    AB3DMusicChange(0, "lventry");
    AB3DMusicChange(1, "cd01");
    ACS_NamedExecute("AB3DMusicFadeIn", 0, 1, 0, 0);
}

Feel free to replace lventry and cd01 with your own choices of course. You should save and then click the blue down arrow at the top of the window afterwards to compile the code. If you get a message with copyrights and Raven Software and such, it's worked. If you get error messages here let me know.

Also I just realised I gave you the wrong code before, it's #import "acs/osiris.acs", not #include.  Ooops! Sorry about that, I was going on memory and its been a bit since I was working on the scripting stuff. Also sorry that such a seemingly simple thing as music choice is kinda complicated, instead of just being able to change the MAPINFO, had to do it this way to get the multiple soundtrack options and dynamic bits, though it's probably a bit overengineered!