That's a cool idea, and I'm pretty sure it's possible too. There would be some fancy math, the car would have to calculate the Doppler shift both ahead and behind it, then intersplice the two audio signals such that they destructively interfere. Now, with the addition of Doppler shifting, I would think that you could broadcast the signal so that when the waves are clumped, they shift into audible range, and when they are separated, they shift into a separate audible range.

After having typed this out, I'm pretty sure this doesn't work but I'm not sure why