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.
