I do something similar, although I don't use a hardware mixer. I use VoiceMeeter Banana, which is a freeware software audio mixer for Windows. A software based audio mixed gets around the fiddly nature of audio hardware I/O on most PCs. It also allows me to use USB audio devices (headsets, microphones, etc)

In my case I don't run any audio through vMix. The audio is all mixed in VoiceMeeter, in parallel to vMix. vMix conveys the final stream to YouTube (or wherever)

Michael

skatejack,

Theoretically that should be possible; sending video and audio over HDMI. While I have not done that, there are couple of options you might try.
(Guessing you want something like crowd noise from the cameras along with host audio with the mixer?)
Bring the host audio in with the mixer as an input.
The HDMI camera video and audio should arrive together on the same input. Make sure the audio is live (green) on the camera input.
If that doesn't work well for you, bring the HDMI camera input in without any audio.
Bring the camera HDMI audio in on a separate input. Be sure all audio inputs are live (green)
Adjust all audio inputs accordingly.

~George

FYI - Vincent Burel, who replied earlier, is the author of VoiceMeeter.

HDMI conveys both video+sound. You can select the HDMI output as an audio destination.

For example, my Lenovo laptop has a mini-DisplayPort output. I can connect it to the HDMI capture card in my vMix machine using a DP-to-HDMI cable. When I do audio from the laptop appears allong with the video as a source in vMix.

I realize that's going the other direction, but the principle is the same. You can pass video+audio over HDMI.

Michael

There are many variables to consider, some based upon the specifics of your computer, sound card, etc. You also nee to consider how practical it is to manage the setup once you have working.

I suggest you try it and see what happens.