There are a lot of things that you need to do in order to enable audio. See several other threads here that discuss the steps:
Basically though you need to:
ensure the proper IO pins are enabled in uboot
create the necessary nodes in device tree
turn on the drivers in the kernel configuration
turn on the userland tools (alsa) in the buildroot configuration
potentially add a new driver source for any external I2S devices
I have tried all these things and still haven’t successfully seen the audio device become available. There seems to be some critical driver that’s missing which prevents the full audio stack from loading correctly during boot. If you figure this out please post here so the rest of us can benefit.