If you look throught the entdata of the 3 .bsp files there are a few references to c3a1 and c3a2, and from sound\sentences.txt:
//DEMO NEWAUDIO
BA_1SAT barney/c3a1_ba_1sat
SCI_2SAT scientist/c3a1_sci_2sat
BA_3SAT barney/c3a1_ba_3sat
SCI_4SAT scientist/c3a1_sci_4sat
BA_5SAT barney/c3a1_ba_5sat...
I think to get rid of the sheen you need to extract all the .vmt files from soure 2007 materials.gcf\hl2\materials\models\alyx\ to steamapps\<user>\half-life 2 episode 2\ep2\materials\models\alyx\ (create the directory if it doesn't exist).
Open the .vmt files in Notepad and try deleting any...
You could copy the Half-Life 2 Alyx from source models.gcf\hl2\models\, but it would probably be missing animations.
I'd try playing with the .vmt files to get rid of the sheen (phong?).
I don't think this is possible without making a mod. I believe there's some code on Valve Developer wiki to properly enable npcs in Half-Life 2 Deathmatch.