Anyone have any insight into better WebM compression?
Take a look at Encode/HighQualityAudio – FFmpeg or the full wiki FFmpeg
It does tend to be lots of trial and error, but you can probably squish the embedded audio down a bit without much bother. One bug I did encounter was when mucking around with the Credits movie (I added Yes back in :E) it actually started to loop a few seconds of the music a few minutes in. Other Webm players handled it fine, so I can only assume whatever libraries the game is using are acting a little bit weird.