Our taps and toilets make a loud horn sound when taps are not open full and when toilets are nearing the end of filling up after a flush. We had a plumber come by who said the PRV outside our unit, near the water meter needed replacing. Its been replaced but we still have the sound. The PRV company replaced the valve again just in case it was faulty but that did not help either.

If the sound occurs regardless of which tap or toilet is used what could be the problem and how do we solve it?

OK start by telling us what the PRV has reduced your pressure down to, what psi. Have 2 friends place their hands on your main shut off, water meter and the PRV, make the noise and tell us which of these 3 fixtures is vibrating, its got to be one of them.

The valve is a 600Kpa Kwikot 15mm valve.
We live in a town house on the first floor so our main shut off is at the metre itself.
The vibration starts at the valve (situated about 0.5m above the ground) and then everything (pipes and taps) from there upwards vibrates with the sound.

And your pressure is..

According to the Kwikot information sheet, the operating pressure is 600kPa.
If this is not what the pressure is being reduced to how do I determine the pressure?

You would either ask the plumber what your pressure was or connect a pressure valve to your outside garden spigot. Your operating pressure would equal almost 88 psi which is way too high but that is the upper limit of the valve setting, we need to know how mcuh the prv brought the pressure down