Cassie could sense that something inside the massive droid was throwing off the echo, distorting the sound, making it much more difficult for her to triangulate the proper frequency to use. She could take that into account, but it would take much longer, and time wasn't a luxury they currently had. It also didn't help that the droid was moving away from the trap of sonic charges that she had laid down, and was instead rolling towards Mako. Of course, for a machine, Mako was the obvious real threat, and it would try to take her out first.  Still Cassie continued to 'ping' it with weak sonic blasts to continue to gather data points, but she knew a different tactic might be in order: subterfuge.

Cassie recalled the news footage of the first omnidroid's attack on the city.  Fortunately, Syndrome really liked to talk, and being a sound genius, Cassie had his voice memorized.  Clearing her throat a bit, Cassie used her sonic projection and called out, mimicking Syndrome's voice perfectly, "OMNIDRIOD. STAND DOWN!" It was a gamble at best.  It was possible that the Omnidroid didn't respond to vocal commands, and even if it was supposed to, there was evidence from prior footage to suggest that the Omnidroid had learned to disobey its creator.  Cassie hoped that it would at least stop long enough for Mako to get out of the way of its rolling.