Since Intel has decided not to officially support Bay Trail and Cherry Trail processors in Linux, and focus on other priorities, people who have tried to use HDMI audio in Linux with mini PCs or TV sticks based on the processors have always had issues. There’s a workaround for Bay Trail mini PCs with Linux 3.16 kernel, but so far I’ve never seen a Cherry Trail system working with HDMI audio in Linux. Luckily, Pierre-Louis Bossart has been working on support, and Piotr kasprzak has tried HDMI audio support with OpenELEC 7.0 and a patched Linux Kernel 4.5.1, and confirmed it to be working on Tronsmart Ara X5 Plus and Wintel 10 Pro mini PC powered by Intel Atom x5-Z8300 processor.
[ 6.417670] had: ******** HAD DRIVER loading.. Ver: 0.01.003
[ 6.421553] [drm] mid_hdmi_audio_register: Scheduling HDMI audio work queue
[ 6.459006] dw_dmac INTL9C60:00: DesignWare DMA Controller, 8 channels
[ 6.485499] dw_dmac INTL9C60:01: DesignWare DMA Controller, 8 channels
If you want to try it yourself, you can download the OpenELEC image with the modifications. If you want to build your own Linux kernel with HDMI audio support apply this patch to Linux 4.5, and enable CONFIG_SUPPORT_HDMI=y in the kernel config.
Thanks to Piotr for the info!