Micb how to enable?

I reference this topic. https://openwrt.org/toh/seeed/linkit7688 openwrt is 19.07

speaker function no problem. but microphone function can not working. when I running arecord -c 2 -f cd /tmp/tmp.wav. I can not detect the microphone bais voltage.

I use the openwrt-21.04 to test it. microphone working right.

I dont know if I need to configure some items?