When it comes to computer audio, well then it is in one way or the other interesting how you power different circuits.
Personally I don't think the power supply in a computer is the main issue of noise there, all the individual components themselves inside the computer generate a lot of noise.
If you have a metal box for your computer, when this box is properly earthed, then your computer most likely won't send out a lot of noise to the environment.
Switch off Wireless LAN if you can, this produces major amounts of noise, theoretically.
If you look at a HDD, even this will produce a lot of noise while operating. (I should believe)
Even the data transfer systems (BUS's) are noisy, I would assume the most noisy thing in a computer is the components that is generating and calculating what you view on your screen.
Most noise are to some degree taken care of inside the computer, or else most things in there wouldn't work properly.
Still your point of view is interesting to me.
Guess if you separate the mains to your computer from your sound system with suitable mains filters or common mode filters of various types, plus do the grounding properly to remove noise, then you should be good.
You must be careful with groundings, if you ground a "noise shield" that is the inner foil or other type of screening, then this must only be grounded in one end.
Otherwise you can get noisy currents between different ground potentials and this current induces noise directly into your signal wires.
Often what is done, Earth(Earthed in both ends), then some insulation, then Screening(Earted one end) Preferable the source end.
Now if you go with screening to many things, always connect the screening shield in the same place, and let the other end float(Be isolated).
This can also be done to block noise from getting out from a cable, entering other nearby equipment sensitive to noise.
IT is a major benefit to stop noise this way, cause it then is stopped from making a high frequency noisy current that again produces a major noise issue and can use the whole earth wire of supply cable as an antenna.
I don't know if this got all confusing and way off what you really where thinking of.