Hmmh, tested once more, w/ kernel 5.10.3-towo.1-siduction-amd64. HDMI sound works w/ boot parameter "intel_iommu=off".
Maybe our machines are different, see my "inxi -ACGM" below. That's all I can tell. Stay safe.
Machine: Type: Laptop System: Dell product: Latitude E6440 v: 00 serial: <superuser required>
Mobo: Dell model: 0803HT v: A00 serial: <superuser required> BIOS: Dell v: A21 date: 02/01/2018
CPU: Info: Dual Core model: Intel Core i5-4310M bits: 64 type: MCP L2 cache: 3 MiB
Speed: 839 MHz min/max: 800/3400 MHz Core speeds (MHz): 1: 839 2: 814
Graphics: Device-1: Intel 4th Gen Core Processor Integrated Graphics driver: i915 v: kernel
Device-2: AMD Sun XT [Radeon HD 8670A/8670M/8690M / R5 M330 / M430 / Radeon 520 Mobile] driver: radeon v: kernel
Display: x11 server: X.Org 1.20.10 driver: ati,modesetting,radeon unloaded: fbdev,vesa resolution: 1920x1080
OpenGL: renderer: Mesa DRI Intel HD Graphics 4600 (HSW GT2) v: 4.5 Mesa 20.2.6
Audio: Device-1: Intel Xeon E3-1200 v3/4th Gen Core Processor HD Audio driver: snd_hda_intel
Device-2: Intel 8 Series/C220 Series High Definition Audio driver: snd_hda_intel
Sound Server: ALSA v: k5.10.3-towo.1-siduction-amd64