After two days of trying to google ways to get audio on the hdmi output on a shiny new Udoo X86 running debian 9.0.0, sheer gritted-teeth determination, smacking the walls of the GUI rat's maze lucked onto the deeply concealed interface.
On the LXDE desktop, the "Sound & Video" -> "PulseAudio Volume Control" menu item has only 3 widely spaced tabs, underutilising the chosen window width, and the "Output Ports" tab offered no management or configuration possibilities. But there are two tiny dark triangles in the corners. Clicking on the RH one leads to an unnecessarily hidden tab, "Input Devices", and clicking again reveals "Configuration". There, in a "Profile" selection box, it is possible to select "Digital Stereo (HDMI) Output". Once selected, it even seems to be the power-on default. (Much to my surprise, given the user-hostile perversity of the devious GUI design, deliberately made unnecessarily narrow, so that two vital tabs could be hidden from the user, without the most tenuous rational reason for doing so.) Granted, the purpose of a GUI is to put access to necessary functions at the end of deep maze rat runs - but invisible secret tabs with double blind access?! I do believe that some of these devs are being paid by Microsoft to paralyse linux. (If not, we know that they were born arse-backwards, and have never turned around.) There is no rational explanation for failing to make all 5 tabs visible. Erik (Who in 30 years of s/w development never let a team member produce crap like that.)