I've experienced similar phenomena in the past.  I'd put
it down to temporal correlations in address bus usage,
leading to an audio frequency spike in the frequency
domain of the EM signal that it emits.  The address bus
is the antenna, and you can't really tie a knot in it :-)

You just need better sound hardware.  Most PC sound
cards are crap.  They are not adequately shielded (in
fact, generally not shielded at all).

One possibility is one of these new-fangled USB->SP/DIF
convertors, and plug the SP/DIF (digital audio) into a
modern amplifier that supports it.  Sape's working on the
driver support for this I believe.

Perhaps USB powered speakers would do the trick.

You could also get a professional quality audio card; I
think there's one made by a company called Turtle Beach
or something; some music stores here sell them.  No idea
if they are Plan 9 compatible.