A couple of thoughts-
1.) Standard practice in most audio applications is to ground shielding
at _one end only_ to prevent formation of so-called "ground loops."
Ideally, all the shielding should be grounded at the same end, typically
the engine or aircraft ground while allowing the shield to "float" out
at the headphone and mic jacks.
2.) Check to see if the p-leads--the ones from the ignition units to the
"mag" switch--are shielded. If not, shield 'em. Keep all your audio
leads as far away from that switch as you can.
good luck!
psl
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