Since switching to Debian Testing full time, I have had issues with HDMI audio not working. Videos would play at increased speed and sound would stutter or be completely absent. Using the stable Linux kernel 4.19 works but now that Debian 11 has been released, 5.10 is now stable.
To cut a long story short, today I found a link on the Arch Linux wiki which said to add
intel_iommu=on,igfx_off to the
GRUB_CMDLINE_LINUX_DEFAULT line in
sudo update-grub and reboot to have HDMI sound on the latest (by Debian standards) kernel.
I hope this helps someone else. For reference, this is on a Lenovo Thinkpad T440p, using the Intel i915 driver with nonfree firmware enabled.