Let's forget about the red and green specs era for a second. 3D Cinema actually began when Dolby Digital Stereo or similar formats became projection standards - this was the point where a soundtrack alone could transport you or leave you dizzy in your seat. The set-up mimics real perception - forward vision and surround sound. And if the soundtrack can give you orientation, the score adds depth: those Imperial cruisers in the opening scene of A New Hope would only have been about a third as long without the music.
With Avatar, vision has caught up again - and I think that sound has been kept deliberately low-key. This BBC Music review of the score makes the point well. James Horner, a skillful and experienced cinema composer, (Titanic, Apollo 13, Wrath of Khan) has written a subtle and effective score for an epic sci-fi blockbuster - this is itself a daring and innovative move.