wrote:

My question: is it okay practice to use the wiring shield as one of
the conductors?

NOPE

In other words, can I use 2-conductor shielded wire
for stereo headphones (left, right, and shield for ground) or should I
buy 3-conductor so the sleeve terminal has its own wire inside the
shield? I ask because both the sleeve and the shield get tied to
ground at the radio end anyway.

If you use the shield braid as a conductor, you negate the shielding
effect and subject the bundle to radio/electromagnetic interference. The
radioheads around here can explain it better than I ever can.. but the
short and curly of it is.. dont do it.


Also, should I use a separate wire for the mic audio, or can all of
the conductors run inside the same shielded wire using a common
ground?

Do what your intercom manufacturer recommends on their wiring diagram.
If you dont have it, get it. My particular intercom has mic conductors
shielded. Make another shielded run if you are wanting to shield the mic
hot and audio conductors.

Dave