Default to Opus for the audio codec

[pfrenssen]

We are still defaulting to Ogg Vorbis as the audio codec but this has been superseded by Opus years ago. Opus is fully supported in all chromecast devices.

[pfrenssen]

I've been doing some tests with Opus but I am hitting this bug: https://trac.ffmpeg.org/ticket/5718

With Vorbis this doesn't happen. Probably we should stick with Vorbis until this is fixed.