Hi. If your audio inputs are separate connection and not in your camera in-->sdi out --->capture card so the sdi vmix input will carry audio together,you need to do routing separate your audio.
How many audio inputs you have? Do you use all the 4 outputs? A way to do this you need is to select an audio bus (other than the master) for any of your audio input you need to capture and then select one output 2-4 from it`s cog settings instead of master to record all the buses. Then select this output into multicorder and wav enabled (if you select mp4 multi/recording and not wav enabled mp4 format can handle 6 channels so at maximum MAB and be aware for this limitation or use vmix avi and then see how to proceed further for post production) and if you need also to record the actual camera sdi out sound,you have to select in multicorder the Audio-->input or if you need the pgm audio for your raw camera record you select the Audio-->master
Also see in the audio inputs the setting called "channel matrix" to do other settings needed. For example if you dont mind to record mono your audio inputs, you can do via the channel matrix, for example the Audio 1 input to be recorded in BUS A left channel,while the Audio 2 input can be recorded in BUS A right channel.
Test and see what fits better for you

Excellent leads. Took some poking and prodding, but I got there.

We stream, with a mix of the board and some room mics, but I also wanted all 3 camera ISOs, and the board mix and room mics as separate stereo stems.

Need to go to Settings/ Audio Output and set Bus A and Bus B to Enabled. It's buried in all the pulldown outputs.

In the Audio Mixer, this should give you A and B checkboxes next to the M under your audio channels.
I sent the FOH board mix to M and A, and the stereo room mics to M and B.

In Settings/ External Output, I had to enable External Renderer to get the pulldowns to work, but then I could select BusA for External 2 and BusB for External 3. I didn't need to select an Audio Device, or check vMix Video/Streaming. I unchecked External Renderer.

In Settings/ Outputs/NDI/SRT, you can pick whatever for the output source. If you pick an audio input, there won't be any video in the MP4 that you're going to throw away anyway, and the files will be tiny. In my case that was Output 2 gets Input 4 and Output 3 gets Input 5, since my audio comes after my 3 cameras.

In MultiCorder, enable all your cameras, your Stream Output (1?) and whatever outputs you've routed audio to. Set the Audio pulldown to Master, which doesn't make any sense to me, and click the WAV File Record box.

When you run MultiCorder, it will make an MP4 and a WAV file for each camera, the stream, and your audio buses. You can throw away the WAVs from the cameras and the MP4s from the audio outputs.

Your use of MABCDEFG is very efficient, but I'm just going to pass these files to the performers, so I break it down to conventional stereo files instead of multitrack WAVs. Make it simple as possible at their end.