Rowby,

Not a hardware solution, but a software one that will work with Win7. If Martin and Michael give it a thumbs up it's good.

These are awesome VU meters.
I use them also to get a realtime indication without the latency of vmix and they are a lot smoother than the vMix ones.

The Darkwood meters monitor the levels of Windows Recording devices, so to use the vMix outputs (Main, A, B, Headphones) you would need to feed those to something like the "virtual audio cables" which effectively take an output (as a virtual Playback Device)and make it available as an input (as a virtual Recording Device). If you have a Realtek audio chip in our PC, then you can usually use the Stereo Mix device to perform this task for one output group, but if you want to meter Main, A-bus & B-bus then you'll need something more than that.

Of course you can also obtain these various vMix audio bus outputs as individual NDI streams, which may be of use in some way (perhaps more in the future?). I did write to Paul at Darkwood to suggest he might like to include NDI support but I don't think he's looking to add anything new. There's an audio meter in my NDI NoAudioMeter application and I also have an 8-channel NDI audio meter (for checking NDI audio levels), so it's a pity that vMix doesn't include all the audio busses in a single NDI stream, which is perfectly feasible and has the advantage that all the channels within that stream are inherently in phase, which is not so easy to guarantee when receiving multiple NDI audio streams.

Have you tried any of the Darkwood meters? They will let you select any audio input device (that shows in the Windows list of Record devices) as the meter source.
That can include USB audio sources, as shown in the attached pic of the Dual PPMs in which the audio sources from a Magewell USB Capture device are included in the list of available sources, and it's the same for the other meters in the range (if you don't like BBC-style PPMs).

PPM-InputDevices.PNG (138kb) downloaded 1 time(s).