That one confounded me for a while. You need to equip the purging wand, then if it's not already there, go into the skills menu and add the source vampire(? something like that, the green box with teeth anyway) ability to your hotbar and cast that at in the vicinity of the shriekers.
You also need to equip it on a character that's not source-muted (ie: had the collar taken off). Theoretically you might be able to do the same with Helm of Braccus (it also gives you source vampirism). However the range is much shorter compared to the wand so it might not be feasible.
If it makes you feel any better, I've also forgotten where I found the working one in my last play-through so I'm off to search for it again! At least things keep on being new and exciting when one's memory is as bad as mine.
You can get a Purging Wand from Radeka or from Braccus's vault which you can find near the pirate treasure. Also if you break the chain binding the dragon before before the cave where you find Radeka, you can give him the wand you got from her, and he will in turn fly through and take out the Shrikers as you get close to them. Third option is to get Braccus's helm