I noticed the same thing yesterday when checking AoE2DE with Process Explorer and ProcMon, they showed high resource usage in AudioKinetic (AoE2DE_s.exe!AK::StreamMgr::SetFileLocationResolver). But I’m not very experienced with those tools, so take it with a grain of salt.
Unfortunately, turning the sound volume to zero didn’t help. I suspect replacing the unit spawn sounds with short silent files could confirm if that’s the actual problem, but I haven’t tried that myself.
There was a recent issue where some mod sounds suddenly stopped playing, which got fixed recently. Maybe one of those changes introduced this new performance issue.
See: Sound Mods Result in Missing Sounds
Keep in mind, AoE2 and the Genie engine heavily depend on single-core performance. A complete rewrite could lead to big performance improvements, but due to high development costs and limited financial incentives, that’s unlikely to happen.
I also noticed continuous file access to the .rec file, but I have a fast SSD, and even putting it on a RAM disk didn’t resolve the issue, so I can rule that out. (Though buffering the file in memory might help improve recorded game playback performance on older HDDs.)