Also, we should include (or have a separate list) of open source
hardware projects for audio integration of car PCs, MP3 players, etc.
I have started an audio board which contains audio level adjustment
and switching, for general purpose use. It also contains other features that
can be optionally soldered in, e.g. ipod charger circuit, and a microcontroller
for performing button detection, etc.
The work is at http://360.yahoo.com/shabaz_yousaf and any ideas/contributions are welcome. So far, prototyping is done, will post up the circuit diagram and details very soon.