Bigger cog on the motor, from 11t to 12t. Aluminum cover version instead of glass window version. A small bit of extra tightening on the bolts. Motor chain cover from geared version (red tight fit). Geared motor instead of chain version. Low volts, 48 or 52.
I have both a loud and a quiet bike, where it is illegal to have one.
Ultimately, the sound doesn't matter, with a quiet motor you will be ringing your bell so people notice you, with a loud motor and squeeky brakes you will have the comfort of that automatically happening.
You think a cop will stop you because he hears the sound? By the time you are in hearing distance he already decided how stealth your bike looks and if it's an eyesore enough for him to be interested.