The power in noise is actually fairly low. Like single digit watts low, for deafening noise.

You are only looking at the energy in the noise itself. But to make that noise there is a ton of wasted energy in vibration, friction from gears, etc.

Noise is a symptom of "slop" in an engine. It might be unavoidable, but it's still a symptom of wasted energy.

Oh, definitely. Noise is often a symptom of wasted energy in a mechanical process.

And even producing noise with a speaker is rather inefficient. You need hundreds of Watt input for single digit Watt output in the interesting frequencies.

The only reason these sounds are being added to cars is because some people want the noise. Some for safety reasons, some for aesthetic reasons. In the latter case, it's no different than music.

There will be a set of circumstances where you will be resorting to beeps to alert pedestrians without engine noise.

Time spent to contemplate whether to beep or not constantly may also be wasted time.

Long before everyone has driverless cars—and not long after cars are silent enough for this sort of thing to matter—we'll have cars that are at least smart enough to know when there are pedestrians nearby and beep at them. (Or possibly even only play engine noise when approaching them.)

Yeah, fuck anyone else not in a car.