Comment 4 for bug 830811

The problem here is that you were using libmp3lame as the audio codec - WebM only supports libvorbis for audio. By using an unsupported codec, ffmpeg has crashed, which in turn crashed Openshot. We are not able to catch any crashes in ffmpeg/MLT before they crash Openshot, so I have added some validation to force the correct codecs for WebM i case they are changed by the user.

The best option is to use the WebM profile under All Formats in the Simple export tab.