Probably the most expensive add-on that people get in their cars today is the stereo. Long ago, cars often came without stereos and there was a major aftermarket. The aftermarket is still here but most people elect for factory stereos which fit in seamlessly with the car and often cost a huge amount of money.

The car's not a great place to listen to music -- it's noisy and you are distracted and you often stop and have to get out in the middle of a song. But because people find they listen to more music in their cars than at home, they often pay huge bucks for a fancy car stereo. (Not counting the people who deliberately buy a system so loud it's meant for other people outside the car to hear.)

While you could put a nice stereo system in a robocar, and some people will, another way they can save money is they don't need to have much audio at all, not once they can do full-auto operation. The prohibition on headphones by the driver should go away, and it could become popular to just use nice headphones -- possibly noise cancelling headphones or in-ear noise-blocking phones. A better audio experience with much less noise, and a lot cheaper too. And there is the option for each person in the car to have their own headphones and tune their own audio stream.

People will like to share, so the car might contain a simple audio distribution system to feed audio streams to people who are sharing, though the source of the music should still be somebody's phone or device, not something built into the car. In addition, there could be a system to mix in some of the in-cabin audio, so you can still hear the other people when they talk. Microphones on each person's headphones could pick up their voices and actually provide a clearer read of their voices. Headphones with position sensors could allow simulation of stereo on the other people. Alternately a microphone array could exist around the car, particularly at each seat.

There are some downsides to push things into the traditional way:

• Wearing headphones is uncomfortable on long trips
• They are a pain to remember to put on. You want to avoid cords, so they would be wireless, but then you must be sure to put them in their charging dock.
• On small aircraft, there is so much noise that everybody does it this way, but they tend to be bulky (due to the high noise) and unpopular for that reason

So people might elect to still have decent speakers and listen to music without headphones. But there is less need to buy a really expensive sound system, since if you want the top quality you probably want to go for the headphones. This may also apply to decisions to do expensive sound elimination in the car. For some, nothing may change, but that's OK. What's interesting is the option to do car sound in ways never done before.