It would be good to have a rigid specification for this kind of thing, same as for the colour and blink frequency of signal lights, road sign designs and number plate fonts. Predictability and standardisation is super valuable on the road.

However, I see no reason why the specification can't specify a particular electronic burbling noise from The Jetsons.

If the sounds are too different from existing mental models, they won't be associated with oncoming traffic?

While browsers can't decide on a format for the Internet I can't imagine what kind a mess the auto industry could come up for custom sounds for cars. But the arguments would be funny to listen to, 2 guys in a pub, the first boasting about how his Mustang is lossless, the other disregarding due to background noise and file size.

What digikata said. The law doesn't require outside noise for your benefit, it requires it in the hopes that the people in front of you will hear you coming and get out of the way.