The submix asset is only available if the audiomixer is enabled. to do this you can find instructions on how to do this here: https://valvesoftware.github.io/steam-audio/doc/phonon_unreal.html#enabling-audio-mixer-functionality
The submix asset is only available if the audiomixer is enabled. to do this you can find instructions on how to do this here: https://valvesoftware.github.io/steam-audio/doc/phonon_unreal.html#enabling-audio-mixer-functionality