The Amulet solves that neatly. However, player freedom means that they're not guaranteed to find the amulet. So in places where the designers fear that players might murder plot points, they'll place a scroll of speak with dead in a nearby chest.
This basically means that there is no need for the existence of such thing as the Amulet of the Lost Voices, at least in its current form. If this spell is necessary for pushing through the story points, then scrolls would suffice.
All of that is fair. I'm guessing that the designers aren't willing to take any chances on players missing key pieces of information.*(see edit) Remember that the power granted by the haunted book is
also Speak with Dead. That's waaay overkill on the face of it, so let's give Larian a modicum of credit and assume there's a reason.
I can see two, entirely speculative, reasons. The first is that narrative payoff depends on player knowledge. If the game spends a bunch of effort setting up a payoff, it doesn't want the player confused out of pure ignorance. "Ok, but why are you betraying me, again? I killed your brother? Listen, I kill a lot of people... Who can keep track?"
The second reason is that it's really hard to get players to do anything. Larian's designers count on players seeking out information from corpses when stuck. That requires 3 things. 1) The idea that it's possible to Speak with Dead; 2) The means to actually Speak with Dead; 3) The willingness to spend ressources to do so. Multiplying SwD isn't subtle, but I'm guessing it's effective in provoking the desired player behavior.
*Edit : this is not at all consistent with the camp scenes system, where it's trivially easy to miss big chunks of the story. So maybe my underlying assumption is flat out wrong. Maybe Speak with Dead is just a placeholder effect for early access, which will be changed on full release.
