Spirit Mediums and Witchoil

I have a question regarding spirit mediums being around witch oil. During adventures one and two, it states that spirit mediums hear screams whenever they are nearby these witchoil iron golems. I believe it was due to the souls in the witchoil. But i'm not sure if it was just because of a special type of witchoil was in the golems or if its from any source of witchoil. It never really says anywhere else where there is witchoil that spirit mediums hear any voices. I have a spirit medium player that is constantly asking if she can hear the screams when they are near any black substance and I'm not sure if i'm telling her the right answer or not. Can a spirit medium hear voices from ANY source of witchoil? Or were the golems unique in this? This was especially troublesome during the Macbannin scene in Adventure two, as the spirit medium player refused to go into 'that' place since I had already said that she heard 'an ocean of voices' coming from there. Also does witchoil have to be 'charged' with souls before it becomes witchoil?

Witchoil is a physical manifestation of the border between the real world and the Bleak Gate. If someone dies near it, it traps the soul and becomes 'charged.' Since a lot of people died on Cauldron Hill, all the witchoil produced there is already charged. If the party found 'wild' witchoil, it wouldn't necessarily be charged.

But yeah, most of the stuff that shows up in the campaign is charged, and spirit mediums could hear voices from it. But that only shows up in 2 with the golem, the flayed jaguars, and Macbannin's lab; in 3 with Kaja Stewart's golem buddy; in 5 at the Cauldron Hill facility; and in 7 with the golems guarding Leone. Spirit Mediums are also good at tracking the colossus in 8 by listening to the drips of witchoil.