You should use the "origin" entity for the damage too. I would just use a script_origin for the sound and damage. That way you wouldn't have to set it to be a dummy model.
random_explode1:
wait (randomfloat 13 + 23)
$random_explode1_origin playsound leadinmp
wait 1
$random_explode1 anim start // entity in the map must have this targetname.
radiusdamage
$random_explode1_origin 256 384
goto random_explode1
end
If you decompile the obj_team3 bsp, You can see that there are two entities for every ramdom explosion spot. The origin one is above the exploder one--about 64 units.
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Nemisis has a map for his tutorial too, but it has no leadin sound.
http://users.1st.net/kimberly/Tutorial/ ... osions.zip