I’m using NDI Hx APP on an iPhone 8 as a video camera in addition to 2 Canon video cameras. The audio out from our soundboard is fed into the vMix PC using a USB Audio Adapter. The Canon cameras have the audio selected to use the USB Audio Device. When either Canon camera is selected, the audio is being played and streamed by whichever camera is selected. I want the audio from the output of our soundboard to be streamed with the video from the iPhone when it is selected. I have added an audio input, selected the USB Audio Device, this causes the audio microphone to be selected. But the mic is open all the time, the audio is being played over the videos that are being streamed. I can turn off the audio microphone by toggling audio, on the audio microphone thumbnail or in the mixer. I'm testing using music from an iPad, simulating our soundboard. I’m unable to control when the audio feed starts or stops when the NDI camera is output window. I must toggle the audio in the thumbnail or mixer to start or stop. I’ve tried various triggers, no luck. I’m having difficulty trying to accomplish this. I can do it manually, maybe there isn’t a way to automate this process. Anyone have any ideas?

If I understand our issue correct just deactivate the "audio follows video" or "automatically mix audio" and leave the soundbar on permanently. You then switch the cameras of your liking.
https://www.vmix.com/help21/index.htm?Mixer.html