Hello Takashi,
According to your application, I think it will be better to follow these steps:
1. Download L4.14.78 documents.
Embedded Linux for i.MX Applications Processors | NXP
---L4.14.78_1.0.0_LINUX_DOCS
then decompress it, documents below will be obtained:

2. Downloading and compiling Linux BSP source code
In i.MX_Yocto_Project_User's_Guide.pdf, detailed steps are described, you can download and compiling i.MX8MM linux BSP according to those steps. ( Linux BSP on my computer is 4.14.98)
(1) Platform driver(fsl-sai.c)

(2) codec driver(wm8524.c)

(3) machine driver(imx-wm8524.c)

Hope above information is helpful for you!
Have a nice day!
BR,
Weidong