How? When I select the monitor's speakers (monitor connected via HDMI) in Windows 10's traybar symbol before I start 0 A.D., I hear sound. When I start 0 A.D. in windowed mode, I hear its sound, too. But when I run 0 A.D. in fullscreen mode, there is no sound anymore - without any changes from me about the output sound interface. The only activated other output sound interface is USB headphones, which always work without problems. It seems that the fullscreen mode causes audio-via-HDMI problems.