Interference can show up on any 25/70/100v system. I know this because it happened at a School district I use to work at.

Brand new state of the art Fire Alarm system installed along with a Paging system and when they were turned on you could hear the data from the fire alarm system on the paging speakers. The paging system required shielding per the manufacture and the installer ran shielded cable, but didn't connect the shields together. Once the shielding problem was corrected, the data noise went away.

So the end result is do what the manufacture of the equipment requires.

Oh, and never use shielded cable for regular speaker cables as it causes all kinds of problems. You really don't even need to use twisted pair cable. If you are running speaker wire in conduit, just use regular wire the same as you would use for an ac circuit. Otherwise standard 2 conductor cable is just fine at the appropriate gauge.

Patrick T. Caezza
Santa Paula, CA 93060
C-7 - Low Voltage System Contractor - Lic# 992448