I really can't tell you... all I can tell you is that al_wt_rainlight1 and al_wt_rainhard1 are not in any of the NWN bifs. For example, place down the Rain Heavy sound effect, open it's properties, remove al_wt_rainheavy1, click on add and search for it. You'll see it isn't there. Maybe they changed how weather sounds work in some patch or expansion, deleted the sound effects from the bifs (and moved them over to the ambient folder), but didn't delete the blueprints .
Much more strange is why the sea surf sound effects (Sea Surf Large and Sea Surf Small) don't work, as as_na_surf1 and as_na_surf2 are actually in the bifs, and play just fine in the properties tab but won't play in the toolset or in-game... and I'm quite sure I remember they worked around the time of patch 1.67 (maybe before that patch or including it, I'm not sure).
However, it still works when you create a custom sound effect with as_na_surf1 or as_na_surf2 (and works as well when you add them to any other preexisting sound effect). Once again
I think I figured this issue out.
Sea Surf small plays for me both in toolset and ingame.
Sea Surf large doesn't. Turns out the sound blueprint has bugged volume, zero (technically 1, but where min is 0 and max is 127 so its like 0). If you raise the volume on the sound object it will play.
Im adding the fixed blueprint into my patch, however it will not fix this sound effect retrospectively as the placed sound objects will keep the volume they had in the time they were placed as all such objects are then saved into area files and no longer uses values from palette. Newly placed sea surf large sound objects will however works as intented.
There also appears to be a typo/overlook in the sea surf small sound object where the interval variation is 0.05 instead of 5.0 which is in al others sound effects like this. I didnt really noticed any change in game after fixing this, but im adding this as well.
Still havent cracked the rain sounds but yes I agree that it might be remnant of the old way how weather worked and bioware forgot to take these sounds away when it was changed.
Thanks. This way I was easily able to compare my folder with yours. Turns out I have a lot of custom sound effects. Shame I was so excited they might be vanilla and unused '>.
There are two vanilla unused, but thats al_fanfare_horn and al_fanfare_trump and none of them suits as ambient sound + both of them are included in BIF, so there is no usage of them at least as an ambient sound (might be useful as normal sound).