You have a few options. As @JohnFromGWN mentioned, you can export the '.rel' files (to '.rel.xml') using CodeWalker's RPF Explorer (always get CodeWalker from that Discord link, one on gta5-mods is out of date). Once exported, search files for 'thunder' & 'lightning' etc & then lower the '<Unk' values ascosiated with them to find the one for volume.
'weather_thunder' in 'categories.dat22.rel' is a global value, lowering the volume there will lower the volume for all thunder sounds at the same time (same for any rain ones you find in there too), but you can likely use the same principle in other locations/'.rel' files to find & isolate individual sounds & lower volumes one at a time.
If you want to edit the actual Thunder/Lightning/Rain audio files you can find most of them in:
...\mods\x64\audio\sfx\RESIDENT.rpf\weather.awc (contains thunder/lightning/rain sounds)
...\mods\x64\audio\sfx\ONESHOT_AMBIENCE.rpf (filter by 'thunder' & also 'lightning' & you'll find their '.awc's)
For '.awc's, there's no way to filter the files inside them, & often files have hex_hash names ('0x06B8CE61' etc) so they won't show up in a search for thunder/lightning/rain anyway. You just have to listen to all the sounds in the '.awc' & figure out which ones are thunder/lightning/rain, make a note of them, & then right-click the '.awc' > 'Export to openFormats' using OpenIV ('.oac' file & folder of individual '.wav's produced) & edit the identified '.wav' files with a program like Audacity etc & finally reimport them by dragging the '.oac' back into original location & OpenIV will automatically convert the '.oac'(+'.wav's) to an '.awc'.
Might be the odd thunder/rain sound in '.awc's like 'ambience.awc' or such. Most are in 'weather.awc', but maybe broaden the search for thunder/rain audio file locations if you find some sound volumes are not affected when you edit the main file locations mentioned above.
You have to use Codewalker to do this properly. Using a combination of Codewalker's ambient audiozone map and Codewalker's .rel editing capabilities, you can either add or remove individual sounds. And you need to do that, since many zones use several layered individual sounds.
To get started go here, go here and download the latest Codewalker version :
You will obviously have to learn how to do this gradually, so experimenting will be a part of your process, but you can always ask for help in #fix-my-mod. Or you can hit me up on Discord : Modifiver#1744
I might take a while to get back you, but eventually I will, heh.
When you mentioned screen getting blurry and textures getting messy, I thought it was just how the pc performed, but now that you said mentioned that you have a high end pc, honestly I have no solution for your case. It should work fine since it’s a high end pc.
@meimeiriver The game update introduced some breaking changes and Menyoo doesn't fully rely on scripthookv but uses his own hooks for certain parts. Iirc Mafins did this during the T2 disaster when everyone thought there will be no scripthookv update anymore.