Pleasant country stroll music is no good for fighting a troll. But it should not change until a hit is exchanged or at least until the creature is in your view screen.
I agree with this. It seems slightly unfair that the PC can sneak up on NPCs and creatures, but they can't do the reverse. Besides, it's annoying when the combat music starts up because a Mudcrab two leagues away has spotted you and wants a fight, but has no chance of ever actually getting to you (or you finding it...).
A more diverse soundtrack would be much appreciated however. Just going from the Hubdates and the general released material, it seems like they have this well in hand however. For example, the description of the concept art video said that the background music was an example of music from the soundtrack. I think it did anyway. Regional music would be nice though. You can tell a game has good music when you can tell what town or region you're in just by listening to it.