Basic Sound Replacement Guide

Guide contributed by Nooto

Prerequisites / Tools Needed

Already have the game files extracted as per the Extracting Game Files (PC) section of this wiki
FusionTools (for opening .bnk files and listening to the .wems inside) _{The link says it's for RE:Village, but both it and MHR run on the same RE Engine and the same(?) file types, so it's fine}
Wwise (I just use the 8150 version for Rise), through the Audiokinetic launcher, which you need to create an account for
Audacity (for audio editing and volume fine-tuning, etc.)
The MHR mods: REFramework, and then either FirstNatives or Fluffy Manager

Audio Replacement Process

(Find and) Grab the relevant .bnk.2.X64 files from the extracted re_chunk_000 pak folder. patch_001 is Sunbreak(?). Note the file structure of where you found the files (e.g. natives/STM/Sound/Wwise)

Example: if you're trying to change a specific weapon's hit sounds like I did with my Hinokami Tanjiro LS mod, you probably want to start your search in re_chunk_000, and go to natives/STM/Sound/Wwise. Using a hypothetical Switch Axe mod as an example, you're probably looking for files that start with pl_wp_s_axe_[...], particularly the highlighted ones in the following image. The pl_wp_l_swd_[...] files are where I found all of the files I needed for my Tanjiro LS mod, for reference.

_{(No, I do not know why there are 9 billion _gimmick_[...] and _one_off_[...] files, they're there for all the weapons. My best guess is that they might be for weapons made from certain monstie mats that have special swing/hit sounds. I found all the sounds I needed for my mods outside of those, though.)}
Remove the .2.X64 file extensions (via renaming the file, ignoring the warning) so FusionTools can detect it as a .bnk
Open the .bnk within FusionTools (If it's only showing .pck files, change the file display on the bottom right on the file selection menu)
(Find* and) Note down the relevant .wem for the sound effect you want to replace. Do not close the FusionTools window yet!

4*: if you don't know which .wem it is within the .bnk file, you have to listen one by one and try to spot it, and otherwise, trial and error by replacing each file and then going in-game to see what changed. It's even worse if you don't know which .bnk the sound is in to begin with.
Depending on how hard the sound is to find, you might need to start noting down what sounds are what as you test them, if it comes down to that. For example, the following image was my personal """""documentation""""" of me testing different .wem files in different .bnk files for my various mods/ideas I was tinkering around with.

Good luck!
Get the sound you want to replace with (in .wav format), and depending on the nature of your mod, maybe combine it with the sound effect already in-game (extract the sound by right clicking in FusionTools and extract as .wav) in Audacity if you'd like
Open Wwise through the Audiokinetic launcher (again, I just use 2022.1.2.8150), select whatever project you wish
Project (top left) -> Import Audio Files -> Add Files -> [select the .wav you want to replace with] -> Import (note that it imports at whatever project file path you have selected on the file manager on the left. if you're currently selecting a file while importing, it'll overwrite that one!)
Right click your new file in the file manager on the left, click Convert, make sure Windows is ticked, and confirm the Convert
Retrieve your converted .wem from Documents/WwiseProjects/[your project name]/.cache.Windows/SFX. I recommend creating a folder shortcut that leads to here
Swap back to FusionTools window with the original .bnk file open
Right click the .wem you want to replace in the FusionTools GUI, click Replace Wem, and then replace with your new .wem
Top left icon -> Save File
(Important) The file will save as a .bnk, even if you specify .bnk.2.X64 (it will save as .bnk.2.X64.bnk). Rename the file to what the original .bnk was called, and make sure it ends in .2.X64 (and is a .X64 file), ignoring the warning once again
For FirstNatives: In your MHR install directory, create a natives folder (if you don't already have one) and replicate the folder path of where you found the original file, then stick your modded .bnk.2.X64 file in there. If you want to upload it to NexusMods, copy the proper file structure starting from (and including) natives, and make sure that the final folder(s) contain only your modded file(s). If your mod has multiple "options" to pick from, upload them as separate files under the same mod page.

For Fluffy Manager _{(I don't use Fluffy Manager personally so I'm less confident about what is and isn't required)}: Replicate the folder path of where you found the original file, starting from (and including) natives, then stick your modded .bnk.2.X64 file in there. Stick that new natives folder into another new folder, which should also contain a modinfo.ini and [thumbnail name].png.
For the modinfo.ini, honestly just grab the Fluffy Manager version of my Hinokami LS mod, then just copy the modinfo.ini (it should have all the fields you'd need) into your own, and then changing all the info in the .ini file (with a text editor of your choice) to your own mod's info.
The file structure should then look something like this:

If your mod has multiple "options" to pick from, refer to my Initial D Lance mod for Fluffy Manager to glean the file structure and additional stuff you'd need in the mod folder, including bundleinfo.ini. The file structure in that case should look something like this:

That should be it! Be sure to test your mod in-game (either through FirstNatives or Fluffy Manager, remember to enable it if you use the latter) to see if it works! Do note that if you test it and wish to make small changes to your mod, like tweaking the volume for example, you're going to have to go through the entire process starting from Step 2 again using your new/tweaked .wav file. Godspeed.

Afterword

If you made it this far, congratulations! I hope this guide has helped ease the amount of secondary research you might've needed to do to reach the same point I have only after painstakingly piecing this process together through bits and pieces. There might've been an easier way to reach that point personally, but in that case then I was too stupid to find it.
Regardless, I publish this in the hopes that my random esoteric knowledge will of use to someone out there.
Depending on if people request it or not, I might write a guide in a style similar to this for changing the Stickers in-game, as I also learned how to do that in an effort to shitpost among friends in MHR. Let me know if so!

If anything isn't covered by this guide, firstly, consult Google. Failing that, the MH Modding Discord might have an answer for you (check the pinned messages in the relevant channels first), you could try asking there. If asking around doesn't solve your problem either, then I'm sorry, may God have mercy on your wretched soul.

Thanks for reading!

Written by ~~Just Some Guy~~ Nooto/Nooto's Bepis

"But Wait This Guide Only Covers SFX, What About BGM?"

I tried learning how to do so at one point, and gave up because it was such a hassle that I decided it wasn't worth the effort personally, so I don't have any instructions here. It's an entirely different beast, because you'll need to know how to do this.

I can only point you once again to the MH Modding Discord. Good luck.