There are still some issues with webms. The sound icon is displayed now, they are scaled correctly, but some webms will go through while others for some reason results in an internal server error, webms that previously could be posted.
Seeing you're the one with the new code, and hopefully a test environment, here's a webm that could previously be posted which now results in an internal server error, which you can use for debugging
https://my.mixtape.moe/ctiebq.webm