mekonbekon Hi there! The ffmpeg.js script is a minimal port of the ffmpeg library which we use to allow WebM Opus audio files to work in pretty much every browser.
It is required primarily for Safari, although it is expected that Safari will support WebM Opus in the future, as it's been chosen as a standard for peer to peer audio/visual for web browsers (think skype in your browser).
It runs on start up to decode audio files (only if required) so it is possible that your preloading is upsetting it, but report a bug anyway so we can look into it.
The increased size is a bit of a downside, but the script compresses quite well (~670KB) so it shouldn't be a problem so long as your project is served over gzip. Also it means you don't have to include multiple audio formats any more to get complete browser support, so once you get past a few audio files you may find the export is actually smaller than what C2 would produce.
For comparison a 2 minutes audio track:
pcm = 21MB
aac = 2MB
ogg = 1.2MB
opus = 1.8MB