...I have never built from Amlogic buildroot but I will try to help... S905X does indeed have a sound codec built in and it's T9015 - you need to enable it in kernel config to work. To make sound over HDMI work you should have this codec enabled or modify device tree to use dummy codec but the latter won't give you sound over analog. That said I think you should start with meson64_defconfig.
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kszaq, thank you very much. You are a lifesaver. It appears T9015 is in dtb, I just need to enable it in kernel. I am using meson_gxl_p212_1g_releasae_defconfig.
