Plok_HR: I think the flaming sword animation confuses it.
I've made a workaround, place this to <HoMM 3 path>\_HD3_Data\Packs, then add it to your plugin list in the HD launcher.
https://we.tl/t-Huqi3zdBLy (link expires in 7 days)
What I can't yet get running is the animation itself (the HD mod either ignores or is confused by the CREDITS.SMK file), but other assets seem to work right.
Good work haha I don't have the free time to test it right now though, but I'll get around to it
Since the HD mod seems to do more than simply change textures I suppose it's possible they actually removed the playback of CREDITS.SMK at all (probably because it was useless in SoD), in which case I don't think an animated background would even be possible anymore. But at least it should be possible to have a static background of the Archangel