I have the same character in many scenes. His movements are the same, but
what he says differ from scene to scene. I wouldn´t believe it was possible, but I have to ask:
Is it someonow possible to target/control the lip sync of that group instance, so that I can change the mouth movements and only that in the different scenes?
This all depends on how the character’s rig is set up. Is the mouth movement controlled by an armature? Even if it is the same armature as controls the character movement, is it the same action for mouth movement and body movement?
Are you using Rhubarb to lipsinc?
You can append different actions into the separate scene files and use that if I understand how you are setting these up if you are using armatures as suggested above. Just be sure to put your different facial movements into a different action to bring in.