Hello, I've also been looking into this, thought I'd register so I can share what I found.
This might be the definitive answer:
I found this link after searching around for a while: http://stackoverflow.com/questions/8650967/audio-over-micro-usb-android-open-accessory
The Android Open Accessory library provides a basic communication channel with devices via USB. What you do with it is up to you.
(It doesn't have any inherent support for audio, and you can't route the system audio over it. But you could write code which outputs a custom audio channel from your app.)
Keep in mind that the Open Accessory library uses a custom protocol, so won't work with things such as a standard USB DAC.
(For that, you'd want to use the USB Host mode APIs: http://developer.android.com/guide/topics/usb/host.html. The same caveats about not being able to route the system audio still apply.)
So: as I read that, it might be possible to make some custom apps that can send certain audio over a USB port to a compatible USB-equipped device. However, since it doesn't use the "analog" feature of the microUSB standard, you can't just solder some headphones to the pins of the USB connector. You'll need some electronics on the other end to receive the USB messages, strip out the digital data, then put it through a D/A converter and amp to get to your ears. The end product would look something like http://www.amazon.com/Motorola-Stereo-SYN1458A-Headset-micro-USB/dp/B001EJL1UE
, but you couldn't use this because the protocol android is building into their USB support is probably different, but I don't know anything about it.
So, if all that is true, you'd need to make custom Pandora or YouTube apps that send the digital audio to the USB port, then have your custom/prorammable usb-equipped headset receive and decode it. Not impossible, but a bit of a project.