I made a tool to save soundposts so I can still play them locally when the file hosts they use end up dead. Basically just combining the video and audio client-side with ffmpeg-wasm.
It worked fine so I decided to also make a tool to help make soundposts (i.e crop/trim video, split the audio into a separate file, and make sure the video fits 4chan's restrictions).
The UI is very rustic (I'm not a web dev) and the ffmpeg settings used are terrible (vp8 for webm, no opus, only 1pass encoding, etc), but it works well enough for my needs.
feo red.github.io/soundpost-tools/