As with most Perth houses, ours is double clay brick with clay internal walls. When doing the internal clay walls of our first house, the builder mortared between the top and bottom of each brick but not the sides. So there was cement between each row but not between the bricks in a row. That house had fairly ordinary noise isolation between rooms.
The internal brickwork of our current house was the same as our first. However the builder used less cement between rows. In many instances there was barely 50% mortar between each row. You could see through the walls in many places. This house had very poor noise isolation.
In a friends recent build, the brickwork was very good. The internal bricks were fully mortared like you'd see in external walls - minus the raked finished. That house has excellent noise insulation.
Are there any standards or regulations regarding how internal walls are built? There seems to be a wide variation. I'd have thought modern energy rating compliance would have something to say.