There are some devices (mostly car audio) which can play iPhone sounds, read music file info and control playback via USB connection. I'm wondering what technology is used for this? Are there any specs or API?
You must be part of the MFi program (which covers iPod, iPhone and iPad) to access technical communication specifications for those devices.
The MFi Program is intended for hardware developers who wish to develop electronic accessories specifically for iPhone, iPad or iPod*. Companies, organizations, government entities and educational institutions are all eligible to apply. Your organization must have a registered legal entity name to apply.
You can review further information on Apple's MFi website.