Here's our current puzzle.

We operate a high end Dell PC on a dedicated 500Mbps fibre line. It runs Teams (currently "classic Teams") and vMix on the same device.
We note that when we add NDI inputs from the Teams call, rather than the audio being captured from the individual participants, the audio is the mixed "active speaker" audio.

I've been testing on other devices and sometimes the audio is correctly isolated, and sometimes it isn't, so the device itself isn't the issue. Also I have tried on two different accounts, meaning that the Teams account isn't the issue. We are currently runnning v26.

Any ideas what could define which audio channel is ingested by vMix?

thanks, that's extremely weird as it isn't always the case. So it seems like the isolated audio is pure luck. How very microsoft.