It is not the GM mode currently and the way things seem at the moment I doubt that it will be officially added. Larian seem to take it personal if you suggest any kind of automation in GM mode. Especially when talking about dialog. They want the GM to be narrating every npc at all times.
In the engine it is possible to write custom dialog text trees and that may not be so difficult to add in but I have not seen anything on adding in custom sounds or music yet.
Of course, I understand that if a GM isn't GM-ing, then this as a feature could hurt the purpose of GM-Mode. Entirely voiced NPCs would be a nice addition, but I moreso was interested in it for some basic backstory of characters for pre-written introductions.
At the very least a simple short line like, "How are doing kind sir(or madam)?" etc To give the GM a frame of mind for the character's voice. For me, it would be easier to remember who is who, when playing around with 50+ different voices, sometimes you lose the initial voice you were going for.
The game has Quest Dialogue trees, but having Dynamic responses to questions would certainly require a GM. I've never much liked doing the "Narrator" GM for dialogue. I much prefer getting into the characters for more fun. Some options with pre-recorded voices would only enhance/help out the experience for the campaign.