Hes not asking for nihilanth's voice. Some of you need to read his post more carefully.
when you stand next to the red portal, you hear many, many voices
He's asking about the voices that you hear near the teleport to Nihilanth's chamber, aka, voices of scientists, aliens, barney, and other sounds from earlier in the game.
Its a bunch of ambient_generics set to really low volumes and are triggered by a repeating multi_manager. If you want to create this effect(which I am assuming you do) make a bunch of ambient_generics, assign them a different name, check the "does not loop" flag, make a multi_manager, have the multi_manager trigger each sound one after another and finally have it trigger itself in the end, check the "multithreaded" flag so it loops itself.
Phew, long sentence.