It absolutely is the straight cut gears. To answer Dolemite question, the main reason to do it is that it sounds awesome!
My Energica Ego sounds very similar to that Mach-E 1400, and it also has straight cut gears.
Just wanted to follow up on this, we have an official answer now. It's not the motors or the gears, we were both wrong. They are military strength speakers to simply make a lot of noise.