I need to know how to enable audio USB via Sony DAC in my Z2 ; I work in Sony too
As product owner for USB Audio in the Z2, I would like to explain about our USB support.
In the 2013 mid- and high-end products (Z, ZL, ZR, Tablet Z, Z Ultra, Z1, Z1 Compact) we support USB Audio from the Android Audio framework, which means that all audio is resampled to 48kHz/16 bits, and mixed in with notifications etc. This means that we have "bit-perfect" reproduction only for tracks with this sample rate originally (and such are not very common) and turn off all system sounds and all sound enhancements (and even then, I cannot guarantee that we are 100% bit-perfect).
In Z2 and T2, however, we have introduced a new mode, "High-res audio over USB", where the audio stream is sent directly to USB after unpacking/decoding (actually, when selected, this applies to all audio data, no only high-res, so internally it is called "Direct" mode).
The only exception is if a DAC does not support a particular sample rate, in which case it is sent to the audio framework in the previous manner - for instance, some DAC's which support 96kHz do not support 88.2kHz, strangely enough.
Anyway, in this feature we support 44.1kHz, 48KHz, 88.2kHz, and 96kHz in either 16 or 24bits using USB Audio Class 1, and also 192kHz and 176.4kHz using USB Audio Class 2 in Asynchronous mode. The file formats can be either WAV or FLAC for lossless compression, or MP3 or AAC (which decompresses to either 44.1 or 48kHz at 16bits). I guess other generic Android audio formats should work too but they are not in scope since I have never seen these at download stores.
One caveat with 192 and 176.4kHz files is that the Android downsampler does not handle more than 96kHz, so if you get such files, they will only play in High-res mode to a capable DAC, not in any other circumstances.
In this mode, since we bypass the mixer, ring signals and notifications etc are not passed to the USB DAC. If you want these in headphones connected to a portable DAC, you will need to turn off HigH-res mode and everything will then work as in the 2013 products.
I will be present in this forum for questions and problem reports when these devices are released. Currently we are testing with a limited number of DAC' and I will post a list of these soon.