In MovieRecorder the audio configuration is set by the AV Preset (obviously). so this is in the AV Presets that you need to configure the number of audio channels you want to record, map them etc...
The idea is that you can have different audio configurations for different destinations.
Create a User Preset
Go to the MovieRecorder > Preferences > AV Presets. As you can't modify a recommended AV Preset, either you click on the + icon to create a new preset, or you can duplicate an existing one. For that, right-click on it in the list on the left and select "Duplicate" from the popover menu, or simply click on the "Duplicate" button at the bottom of the pane:
Modify the Audio Settings
Click on the "Audio" button to open the Audio settings pane
In these Audio Settings you are able to change:
- the number of tracks that you want to record.
- the settings of each track.
Change the number of Audio Tracks
- Click on the + icon at the bottom of the list to create additional tracks,
- Select a track and click on the - icon to delete it.
Change the settings of each audio tracks
If you click on the little "wheel" on each track to change these settings:
- Codec: Linea PCM or AAC
- Sample Size: 16, 20, 24 32
- Little or Big Endian
And if you click on the other "list" icon, you can change:
- the number of audio channels that are recorded per track and for each channel:
- change the mapping: select which channel from the source (your video device) must be recorded in that specific channel
- change the label: Left, Right, Center,...
For 5.1, simply delete the second track, and set the first track to use 6 channels. That should be it... If you have some specific mapping and label, you can change it there too.