We received stock yesterday and have been testing.
* Using the hardware media player app:
Everything we've tested passes through HDMI. Settings - Playback - HDMI - 'HDMI'. So DTS, Dolby Digital, DTS-HD MA, Dolby TrueHD all play normally using an receiver that can decode these streams. We don't have the facility to test ATMOS.
By changing Settings - Playback - HDMI to 'LPCM' everything is downmixed to 2 channel stereo and passed as PCM.
*Using Kodi (either pre-installed 14.2 or 15):
Regardless of changing settings to 7.1 channel and enabling passthrough everything is downmixed and passed as 2 channel stereo. I presume Kodi is doing the downmix internally. 5.1 channel DTS or Dolby Digital is either passed through or downmixed according to how your Kodi settings.