>>21730605do your fuckin ffmpeg reps, skrub.
ffmpeg -i "locally-saved-webm.webm" -i "https://URL to the sound from the soundpost filename" -c copy out.webm
IF the sound is .ogg or .opus. If it isn't, just change the "-c copy" to "-c:v copy". the audio will get converted to ogg with whatever the default settings are so it can mux it to webm for you. Hell, if you're feeling lazy, the first -i can be the URL to the webm on 4chinz. ffmpeg don't care...unless it tries to hit you with that stupid captcha, then it might care. example:
ffmpeg -i "https://i.4cdn.org/vt/1648994292799.webm"; -i "https://files.catbox.moe/u67s4n.ogg"; -c copy mikoCumming.webm
Input #0, matroska,webm, from 'https://i.4cdn.org/vt/1648994292799.webm':
Metadata:
ENCODER : Lavf58.17.100
Duration: 00:00:13.22, start: 0.000000, bitrate: 1821 kb/s
Stream #0:0: Video: vp8, yuv420p(progressive), 480x640, SAR 1:1 DAR 3:4, 60 fps, 60 tbr, 1k tbn, 1k tbc (default)
Metadata:
ENCODER : Lavc58.20.103 libvpx
DURATION : 00:00:13.217000000
Input #1, ogg, from 'https://files.catbox.moe/u67s4n.ogg':
Duration: 00:00:13.22, start: 0.000000, bitrate: 131 kb/s
Stream #1:0: Audio: vorbis, 48000 Hz, stereo, fltp, 128 kb/s
Metadata:
DURATION : 00:00:13.231000000
encoder : Lavf58.17.100
Output #0, webm, to 'mikoCumming.webm':
Metadata:
encoder : Lavf58.76.100
Stream #0:0: Video: vp8, yuv420p(progressive), 480x640 [SAR 1:1 DAR 3:4], q=2-31, 60 fps, 60 tbr, 1k tbn, 1k tbc (default)
Metadata:
ENCODER : Lavc58.20.103 libvpx
DURATION : 00:00:13.217000000
Stream #0:1: Audio: vorbis, 48000 Hz, stereo, fltp, 128 kb/s
Metadata:
DURATION : 00:00:13.231000000
encoder : Lavf58.17.100
Stream mapping:
Stream #0:0 -> #0:0 (copy)
Stream #1:0 -> #0:1 (copy)
Press [q] to stop, [?] for help
frame= 1 fps=0.0 q=-1.0 size= 5kB time=00:00:00.00 bitrateframe= 436 fps=0.0 q=-1.0 size= 1536kB time=00:00:07.25 bitrateframe= 793 fps=0.0 q=-1.0 Lsize= 3155kB time=00:00:13.20 bitrate=1957.9kbits/s speed=14.1x
video:2933kB audio:207kB subtitle:0kB other streams:0kB global headers:4kB muxing overhead: 0.491939% 
