There don't seem to be any sound interfaces registered with the kernel. Some low-level (e.g. I2S and DMA) modules are loaded, but there's no higher-level ALSA or OSS driver to make use of them.
I can't say anything about audio over hdmi, since I only used it on a monitor without speakers yet, but when I plugged in usb headphones they where automatically detected and used by pulseaudio on debian 9 xfce image.
audio via hdmi does not work. You can use usb audio card. For example from raspberry pi or from odroid. You can find it on raspberry pi store or odroid store.
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