Right ok thanks it works.
In the script file you need indeed enablespeaker when attacking team has obj and disablespeaker when axis has returned their obj.
Sample:
trigger axis_object_stolen
{
wm_set_main_objective 1 1
enablespeaker taken
}
trigger axis_object_returned
{
wm_set_main_objective 1 0
disablespeaker taken
}
First I created 1 speaker via /devmap mode in map
/editspeakers
/dumpspeaker
/modifyspeaker
put in the correct settings :
noise "sound/vcar10/taken1.wav"
origin 2858.02 -1488.28 -1331.89
targetname "taken"
looped "off"
broadcast "global"
volume 250
range 10000
}
}
I think it speaks for itself 
And then when allied team has taken objective, you hear the suspense loop over the “whole” (tweak the range a bit) map.
Then W:ET generates an .SPS file that puts in your mapspeakers.
So with that technique, you won’t need radiant to bsp to test or so.