Desired Outcome: Permit the vMix callers to speak to the host in studio while the show is in standby. This is when there is a countdown timer streaming and output on Master. Once the show starts and a trigger changes the output to Master to the studio only, I want the callers to go to Master too (they are also on Bus A for talkback which works fine)
I have set triggers to that place the callers on Bus B via VideoCallAudioSource and have set the production mic (not the main host mic but that which is used for talk back) to Bus B via the trigger AudioBusOn

On Transition out I have the callers VideoCallAudioSource set to Master (M) and mute the production mic
The callers and studio can hear each other and go out to the stream on transition out without issue but they tell me they cannot hear each other while in “standby”. In Audio Mixer the levels are showing as green.

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I disagree. First figure out how to achieve the desired outcome manually. THEN worry about troubleshooting triggers or scripting.