IoTGateway/BSP/Android/Gettingstarted/How to switch HDMI Audio
From ESS-WIKI
When HDMI connected, the default audio will play to HDMI only.if you want play audio to codec at the same time,Power on, Starting in Android：
shell@rsb4680:/ $ su
root@rsb4680:/ # setprop persist.dual.audio true
root@rsb4680:/ # reboot