RK3399如何设置默认的声卡呢

Firstly use this to define sound cards
pactl list short sinks
You will get a list as;
module-alsa-card.c        s16le 2ch 44100Hz        SUSPENDED
1        alsa_output.usb-Blue_Microphones_Yeti_Stereo_Microphone_REV8-00.analog-stereo        module-alsa-card.c        s16le 2ch 44100Hz        SUSPENDED
2        alsa_output.usb-FiiO_DigiHug_USB_Audio-01.analog-stereo        module-alsa-card.c        s16le 2ch 44100Hz        SUSPENDED
3        alsa_output.pci-0000_00_1f.3.analog-stereo        module-alsa-card.c        s16le 2ch 48000Hz        RUNNING

Then select you have to use hdmi and to set it use this command;
pactl set-default-sink 'alsa_output.pci-0000_00_1f.3.analog-stereo'

open  /etc/pulse/default.pa  and change the related lines as;

### Make some devices default
set-default-sink 3
set-default-source 3

Pls use index numbers and names as your output values.