I was getting this error when I updated xcom files to 1.9, and at first I tried deleting some rulesets and it worked, I pinpointed the problem down to the Units ruleset and it seems these entries make the mod not work, when I deleted them the game works, Im guessing its because these should go somewhere in the armor section?
Update: I added a "fixed" version of the ruleset, It incorporates ONLY the drone and Cyberdisc sounds found in the Units ruleset back to the armor ruleset, the rest of the tanks you already have the sounds integrated so I only left a textfile with the unused ones removed from the Units one
Ruleset.Rar contains the fixed ruleset files