nauKith hey sorry for the late reply!
For the way to get the webm to work i converted it from an mp4 using ffmpeg! I used this because the webm has to pass through a few requisites for it work properly (library, framerate etc.)
If you want to know what to use, I used this:
-s 1280x720 -vf scale=1280:720 -c:v libvpx-vp9 -crf 20 -b:v 0 -pix_fmt yuv420p -r 30 -an -f webm -map_metadata -1 zm_mapname_load.webm
As for the audio, I did use bo2 sound studio. I first got the wav I wanted to use, and coverted it to flac using https://www.freeconvert.com/wav-to-flac
Next, find the audio file you want to change, and open the sabs in bo2 sound studio.
You should see something like this

Find the audio file in there that you specifically want to change. Mine was zm_tomb_load_lr.SN65.pc.snd
Take that name, and rename your flac file to that.
Then, right click the audio file in sound studio, and click replace audio.

Find the audio file you want to use that should have the exact same name now, still as a flac file.
After youve replaced it click file, save.
It should work now!