Hybrids, like the Prius, are very quiet when using only the electric engine – such as when they are backing up. This can pose a hazard for pedestrians. So automakers have proposed adding sounds.
Nobody’s going to want just ordinary sounds. I can see this becoming like ringtones, with everyone downloading a specific song of their choice. I can see “Born to be Wild” being very popular, or “Pink Cadillac”, or that old Chuck Berry song that has the lyric “Cruisin’ and playin’ the radio … with no particular place to go.” That one has a great guitar solo.