{AHS}SONIC's quick tutorial on scripting music into your maps.
(thanks to Pkmnrulz240/Dark Cloud for the original tutoiral)
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There are multiple ways to do this but well go with scripts for now.
Basically, we can use ai to play music. this will not synch through the
netcode,meaning if you put this script in a multiplayer map your oponent wont
be able to hear it when you activate the script. 

1. Create a full, working ai encounter* named "tutleets1" (no quotes) for simplicity.  

2. make sure the squad is named "uber" for simplicity (no quotes)

3. add the following code into your maps script folder in the data/levels/{your map} 
directory.

4. put the sound tag folders included in this package into the following folder:
Halo custom edition/tags/sound/test (if there is no "test" folder
then go ahead and make one.)

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(script startup musicnai
(ai_place tutleets1)
(sleep_until (= (ai_strength tutleets1/uber) 0))
(sound_impulse_predict sound\test\h2test1 true)
(sound_impulse_start sound\test\h2test1 none 1))
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NOTE: you can change the h2test1 part to any of the other tag folders
included with this package.

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For getting the sound_looping tags included into the map, simply
edit the script:

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(script startup musicnai
(ai_place tutleets1)
(sleep_until (= (ai_strength tutleets1/uber) 0))
(sound_looping_predict sound\h2test1)
(sound_looping_start sound\h2test1 none 1))

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The basic function of this script will play some halo 2 music whenever you 
completely eliminate the encounter in the game. In other words, It'll play
some awesome music when you kill all the elites. However, if you use the sound_looping
tags, the music will continue to play after the elites are all dead. 

Another way to do this is to apply the .sound_looping tags to the portals
or clusters in your map. You can see how to add .sound_looping tags in the
HTML tutorial that comes with the Halo editing Kit. (microsoft games/halo custom edition/Hek tutorial)


* you can see a very helpful video tutorial on adding AI 
at the following link:

http://hce.halomaps.org/index.cfm?pg=3&fid=2136


see contact info for any questions or troubleshooting