We have been through this exercise.
As far as defects are concerned, a building is defective if it fails to meet the BCA’s Deemed-to-Satisfy Provisions.
I can tell you that as far a noise is concerned, the BCA’s Deemed-to-Satisfy Provisions (BCA part F5) are pretty low.
You can have have consultants and engineers check the building. If the building passes the Deemed-to-Satisfy Provisions then any changes you make to improve noise insulation would be considered improvements rather then defect remedies. They still might be worthwhile but an OC might not be compelled to make them (nor would the original builder).