Even cheap headphones can attenuate noise at around 30dB. That's gonna block 99.9% of the noise. The article makes it fairly clear they are a solution, but a temporary one.Regarding the noise point, I can't comment on other countries but hasn't it been quite normal for personnel in British armoured vehicles to need to wear headphones with mics due to the noise from the engines? Testing hearing would make sense to make sure they were working properly. The point of concern would be if the headphones weren't effective. The article is silent on that.
On the rest, I guess either those are issues that have been resolved or will need to be before the vehicle can be accepted.
Sound noise has the same mechanucal properties as other forms of noise such as material vibrations. And the article also makes it clear there are excessive vibrations from the engine, felt across the vehicle.
The fix could be relatively easy. The source cannot be touched, but the vibrations can be attenuated. Fix the vibrations and you also fix much of the noise.
As this could mean reshaping of some components, this may entail recertification of either components, or in the absolute worst case, the whole vehicle. So basically bureaucracy.