Bueller isn't here, but I am.
So you want to kill the monorail sound? Fine, here's what you do:
Open up the front motorized front car unit by turning it upside down and unscrewing the five Phillips head screws that secure the top section to the bottom. These pieces are connected by thin wires, so be careful when you open it up to not break them.
Looking at the bottom section of the car, you will see a black gear box with the electric drive motor at one end. At the other end you will see a round housing with a cover that is held down by screws. That contains the sound system speaker, where we need to do some simple surgery. Undo the screws to gain access to it.
After removing the cover, find and clip the two wires going to the speaker and cover the ends with electrical tape. You can then remove the speaker if you want, or just re-install the cover over it.
Re-assemble the car by carefully aligning the top and bottom and re-installing the five screws.
Voila, you have disabled the sound!