I work a lot with vMix and I would really like to be able to test each audio output simply by sending a test signal to each output independently. Maybe add a 1kHz tone with a volume control and a small button to send that to each audio output (maybe a button by the side of each route).

Is this not already possible by creating a 1kh Audio Tone as an Input. Then simply sending that to any MABCDEFG place you want?

What am I missing?

I don't know if this would be useful, but I have a free NDI multi-channel audio signal generator app (8 channels as 4 stereo pairs). Each pair can be enabled/disabled independently, and each channel can be a different frequency (when set to multi-frequency mode), as an aid to channel identification. It also briefly drops the left-hand channel every 3 seconds to help identify left/right. (Or it can output GLITS, if you know what that is.)

How I would use in within vMix is to add it as an NDI input and then set the audio channel matrix to send NDI ch1/2 to Bus A L/R, for example, and ch3/4 to Bus B L/R, ch5/6 to Bus C L/R, etc. You can then enable/disable to outputs in the NDI app as and when you want test tone sent to a specific bus output. If you need to feed more than four busses, run another instance of the NDI Tone Gen and you can set up another input with a further 4 channel pairs.

Example setup is shown in the attached picture.
ToneGen2AudioBusses.PNG (121kb) downloaded 3 time(s).

+1