Set your samplerate in the settings. Just below set channels to stereo.
All stereo delivering sources (prepared videos or live input) will come thru in stereo, too.
You will see it in the audio mixer, too. Each channel has _two_ thin green bars showing signal.
By the way: Follow the hint to _not_ directly record to mp4. Use mkv instead. Under File->Remux you can finally quick change to mp4.
