No sound through USB-C to HDMI adaptor on MacBook Air 2019
After trying yet another adaptor (the Anker mentioned in the comment by IconDaemon) without success, I restarted the machine to reset NVRAM. It fixed the issue. In System Preference, the Sound panel now recognizes external devices, and sound goes through HDMI regardless of the adaptor I’m using (StarTech, CLDAY or Anker).