I'm sure you already know, but:
The default device has no passthrough capabilities, but support 7.1 channels PCM:
Code
Device 1
m_deviceName : @
m_displayName : Default (NVidia CK804 NVidia CK804)
m_deviceType : AE_DEVTYPE_PCM
m_channels : FL, FR, BL, BR, FC, LFE, SL, SR
m_streamTypes : No passthrough capabilities
The only passthrough device is the S/PDIF device with 2 PCM channels (5.1 via AC3):
Code
Device 4
m_deviceName : iec958:CARD=CK804,DEV=0
m_displayName : NVidia CK804
m_deviceType : AE_DEVTYPE_IEC958
m_channels : FL, FR
m_streamTypes : STREAM_TYPE_AC3,STREAM_TYPE_DTSHD_CORE,STREAM_TYPE_DTS_1024,STREAM_TYPE_DTS_2048,STREAM_TYPE_DTS_512
I'm afraid as long you do not find a physical connection and kodi configuration to output hearable sound via the S/PDIF interface (like TV(AC3) from Audio quickstart guide - Official Kodi Wiki) there is no passthrough support.