AIUI, the sounds for forward control are synthesized in real time to account for speed. The pitch and the volume increase proportionally with speed until cutoff.
A custom VSP sound can be done easily with a completely separate synth & amplifier that gets info from the speaker wires, or the canbus, or wheel sensor, or gps.
https://mikeisted.wordpress.com/2015/05/07/electric-vehiclerobot-sound-synthesiser/
WRT the sound of quiet ICE vs EV... at slow speeds, the EV can be deadly quiet. ICE machines, with few exceptions, have accessory belts and vibrations that can be heard by the visually impaired, whose hearing is generally more acute. Above 25mph, EV tire noise will sound as loud as ICE tire noise and the VSP is moot.