Only the sound is needed
Nothing has to be converted first. The file is uploaded as it is, the audio track inside it is read, and the picture is never touched or sent to any model: it adds nothing to a transcript and would only make the process slower and more expensive.
That holds for every container that turns up day to day: MP4 from a phone, MOV from an iPhone, MKV from a screen recording, WEBM from a downloaded meeting. If it has sound, it gets transcribed. And if it doesn't, we tell you before spending a single minute of your quota.