The conduit description was a bit alarming, no pun intended, until I realized you probably meant the 1 1/2" was for your stuff, and power conduit was separate. Right?

We always talk about the theory of keeping wires that might cause interference separate from our wires, but in the real world here I don't remember anybody ever discussing problems.

However, whenever I go looking for signals with a toner wand, I hear all sorts of crap radiating from any alarm wires I run across. I believe I'm hearing that "communication" you're talking about. Is it even an acceptable idea to run alarm wires alongside your A/V wiring?

If it's not (all right, now I'm just making stuff up), then you could have a separate panel in the outbuilding that outputs relay connections which are sensed inside the house at the main panel. That way there would only be DC going through the conduit until a relay is tripped, at which point a momentary click is the only possible noise that could radiate from the wire... which could be shielded. Or yeah, CAT5 could probably be used. Sometimes I think it's better than shielded wires.