The Empress has a Voice because she sits on a Binding Chair • /r/WoT
Sammael off-handily mentions the Binding Chair when talking to Sevanna, but he does not know where one is. Of course he has not been to Seanchan where the Empress (may she live forever) sits on one. I think The Crystal Throne is a ter'angreal that allows a non-channeler to weave compulsion on anyone, and this fundamentally effects how society has formed around the Empress...
An interesting theory. Regardless of whether the Crystal Throne is a binding chair, or if its effect qualifies as Compulsion, I like the idea that the Empress has a Voice because speaking with her own voice from the throne has extra effects.