[TB-RK3399ProD] Set the system default sound card under Debian10

1. Background

Develop on the TB-RK3399ProD core board (hereinafter referred to as the board).
I need to transmit the alarm information in the board through the earphone jack (the board does not have speaker hardware!).

In the actual test, I found that the audio is transmitted through the HDMI interface of the board by default:
when it is connected to an external screen that supports audio output, the audio can be played.

This requires additional screen cost, and I need to output audio through the headphone jack of the board.

The system environment of the board is consistent with the following blog:
TB-RK3399ProD] Debian10 uses USB drive-free camera
https://blog.csdn.net/jn10010537/article/details/114435438

2. Check the sound card and audio output device

2.1 Check the sound card through /proc/asound/cards
As shown in the figure below, two sound cards can be seen on my board:

toybrick@debian10:/proc/asound$ cat /proc/asound/cards
0