Virtual reality apply music to different parts of the scene based on where user is looking at.

Hi guys. Im recently working on a unity project which is kinda complicated comparing to the fact that is my first time working with unity. Im doing an virtual reality android app which can be used through google cardboard. I have been told to enter music on the scene, different music based on where the user is looking at, ex. if the user sees the trees he hears the birds singing sound and so on, and sincerely i dont know where to start from :/ I really need heeelp.

PS: sorry for my bad english :P