[Portal 2] (sound/particle) manifests baked into maps don't seem to work in workshop maps.

[Trico-Everfire]

When loading a workshop map that uses custom soundscripts (and likely particles) they will not load likely due to workshop folder paths (this is merely speculation).
maps/<mapname>_level_sounds.txt as well as maps\<map name>_particles.txt are meant to provide a way for custom maps to add custom sounds/particles, but do not seem to function.

[vrad-exe]

The particles one should work since the february 2021 update, unless Valve managed to break it again in one of the recent updates