We already have issues with hackers getting "into" cars and a few "car viruses" have been found. Imagine your car's sound generator (i.e. PA system) being rickrolled at 80 dB. But I'm waiting for someone to combine a virus with GPS data so it can play completely inappropriate sounds based on location. A mild example would be the U of M Hail to the Victors in the vicinity of the OSU campus. Racial or gang relates slurs could be more serious.
I think the sound should change in different areas. In sub-urban area, it could be easy-going music with low volume; in downtown, shopping malls, it could be high volume advertisement. The sound content should be able to be wireless transferred to vehicles from the neighborhood server.
I agree with many posters here that tire noise is sufficient now -- and will be even more noticeable in an EV-dominant near future. We certainly should NOT be producing unnecessary noise pollution, just as we should minimize light pollution, etc. The blind can be given vehicle proximity sensors.
How about going back to that inverter whine that the very early transistor PWM controlled battery driven cars had? Sort of like the whine from some electric fork-lift vehicles? Of course those sounds may take some getting used to, but they could be quite simple to produce, just lower the chopper frequency.
Other than that, they could copy that well tuned exhaust note from the older "Grand Am" vehicles.
The audio warning system should be integrated with one of the dashboard cameras that Yoshida reported on the other day, plus a Raspberry Pi (or similar) processor module. The processor takes the video feed(s) and detects and characterizes nearby pedestrians, cyclists, and animals (don't forget the animals!), then selects from a library of user-customizable audio tracks. Voila - the right sound for every situation. Of course, if the system is too user-customizable, it will be banned within six months once certain drivers figure out how to misuse it.