What's the range of nfc? Would you deliberately have to swipe into a room to make it work?
Normal range is between 2cm and 6cm so if you added a token to a door frame you could possible walk next to it with your phone in your pocket, all you have to do is remember to do it.
Maybe some sort of light beam that gets broken when you enter a room (like those things in lifts (elevators) and on supermarket tills), this triggers something to check if it's you in the room. Maybe via bluetooth signal on your phone, if it believes you're in the room then audio is switched.
Or those things that make automatic doors open, not sure if it's radar or what it is.
PIR sensor would do the job there. Like I mentioned for Jon's automatic lights.
Could link PIR with bluetooth (or similar)
If you had bluetooth you wouldn't need the pir sensors.
Just a script to detect what bluetooth devices are in range, and a bunch of bluetooth receivers scattered around the house to track where you are.
Would need your device to be visible at all times to make it nice and quick though.
Tracking would definitely be smarter. I guess an advantage of PIR would be an instant detection when entering a room.
But if it doesn't know who you are then it's not really much use.
That's why I suggested PIR and Bluetooth. PIR to detect a person, then Bluetooth to see who it is - maybe based on if the person has a Bluetooth signal great than X dB it gets switched.
But the bluetooth being there (over x dB) would surely be enough to do both.
There are also distance sensors for Arduino/Pi that would work too. It could actually detect your movement and allow you to bring the room you are moving to up naturally.
The way I'm thinking about this is a ride/thing at Disney where you might walk but the audio just follows you as you move around.
What would really kick ass is to eventually have this working super smooth and then get those directional, pinpoint speakers. With multiple sets of speakers in a room it would allow multiple people to jam the fuck out and not even hear the others music.
Dunno. Certainly not thousands and as cheaply as possible.