I think Steve Jobs was noise sensitive as he hated noisy systems. He fought tooth and nail with his engineers to make each Mac system he over saw as quiet as humanly possible. Jony Ive appears to be following Steve's lead here with the current crop of systems.
With that said the Mac systems are the quietest computers out there (they have set the bar for quite a few years now). But, alas they do have fans and that could still be hard on someone with Hyperacusis.
Maybe a better direction for you would be an iPad. It doesn't have a fan or even have a noisy keyboard or mouse.
There is little one can do here to modify the system to make it quieter without effecting it's cooling. There are a few things that still can help here:
- Using a SSD Vs Hard Disk will make less noise and heat so if you are planing on buying a new system get one with an SSD or see about swapping out your current HD for a SSD.
- Buying the highest performing system you can. While that may not sound intuitive, a systems processor that has to work harder makes more heat so a processor that makes easy work of the task will run cooler. With the end result a quieter system.
Both of these only go so far and there is nothing more you can do.