Ideally it'd be something older, since serving audio over a
lan needs a trivial performance level. That means lower
cost and lower heat without wasting $ by underclocking newer
parts. It also means it would fit in a smaller chassis with
lower airflow requirements, cooling needs, and thus, lower
noise levels. You don't mention exactly how it's a server
and attached to the HiFi but I'm guessing the primary
requirement will be either dolby audio out or good analog,
meaning you may want to avoid integrated audio for the
latter.
Todays' heatsinks are so much larger than yesteryears' that
even the poor cheap ones can keep anything Pentium 3 or
older, completely silent with a fan throttled back to be
barely spinning. By chosing a CPU with higher FSB than 66,
you could take something like a Celeron 800 or Pentium III
and underclock to 66MHz FSB, and undervolt it if the mobo
allows it. Reducing FSB (and memory bus) also reduces
motherboard's power consumption, there is potential to end
up using even less power than a Via Epia type board because
it uses 133MHz FSB for video performance, yet you need no
video performance. Even so, integrated video will often
keep power and heat lower, or choose one of the older video
cards that didn't even have a passive heatsink. For example
an ATI Rage Pro Turbo or Rage XL runs so cool it's almost
amazing, even many (even) older cards ran hotter.
With that kind of setup, a single fan in the power supply,
running VERY slowly, can keep whole thing cool. Primarily
you'll want air intake to flow past the HDD, so keep that in
mind when choosing a case, or seal all other potential air
intake areas and leave only an opening right in front of the
HDD bay. Also see if you can put a duct on the bottom of
power supply, if so then you might not need the CPU heatsink
fan at all. Taking something like a Celeron 800, running at
533MHz and 1.45V, you shouldn't need a heatsink fan if you
have a decent passively designed (tall, space between tines
instead of long thinly space fins) heatsink.
As for more modern CPUs, I think
http://www.tomshardware.com
has an article (somewhere) about underclocking a mobile
barton to something like 5-7W. Basically you'll be wanting
the CPU to run in the neighborhood of 300MHz at least, but
if your choice of OS supports ACPI HALT-cooling, a
significantly faster CPU will idle quite cool too, but if it
were to get stuck in a busy loop it would be a lot hotter,
so minimalistic cooling plans based on idle heat aren't
always foolproof... but if all it's doing is playing an MP3
or actually serving files over a lan, certainly it will be
idle most of the time unless something else is eating up CPU
cycles too.
Aren't there networked audio players now though? They may
cost quite a bit more but should also be much smaller, a;sp
quiet, and use even less power. Have no idea if they have
decent analog out though, if you need that feature.
An alternate way of looking at this is to determine what
case you want to use, and what it'll accomodate. Or,
consider the budget instead, many different CPUs can be
underclocked, but perhaps the early P4/Celeron is the
hardest since it starts with FSB that may be as low as the
motherboard supports and no multiplier change options.
A K6-2 is also a fair choice if you can find a socket 7
motherboard that gets along with your chosen sound card.
However most of the cases for those were relatively large
compared to some of the mATX and miniATX options available
today.
SO to a certain extent we need know more about what you're
doing with the box, the budget, and what access you have to
(which) older parts.
