I've read this at a Halo FAQ
Can I import/export Halo's bitmaps or sounds?
No. The retrieval and insertion of data is currently not possible. It is being worked on and you may see some results in a few weeks. If you really want sounds and textures, I suggest buying Halo PC and waiting for MOD support, you should easily be able to extract and import data then, depending on how it is set up.
So I doubt it that you can export sounds of HALO in the first place. What you might try is recording them with a microphone, though you'll lose much quality... If you do so, just import the sounds into your map, and change the paths to the paths of the soundset you want to overwrite. If you don't know how to make a soundset, you might want to check out a sound tutorial... (don't ask me where to find any... With a good google search you can find it in 5 minutes or less. Oh, and you might want to check out this site or wc3campaigns.com first...)
(If you can't record the sounds because you don't have HALO, then I suggest not making HALO maps without having the actual game...)