In the Goldsmithing class quests, when you turn in your items for appraisal the guildmaster will often talk about the aetheric properties of the item you've created. Two of the quests involve Silver: The level 25 quest "Or Ever the Silver Cord be Loosed", and the level 30 quest "Serendipity Now". I'm not currently at home, and can't check the cutscenes, but if there is some mention of Silver being potent against Voidsent, it's probably in the cutscenes for one of those two quests.