I have vMix connected to our crestron system, via hdmi. The video goes through but the audio doesn't.

When I select the audio bus A it gives me the crestron option to select and shows the audio bar moving, but no audio is coming out.

If I go to the computer and sound > devices I can test the crestron and it does play the test sound showing it's connected.

If I change the PC sound output to crestron then the audio plays correctly, however I need to control it from vMix rather than send all audio to crestron.

Why would vMix not send sound, but the PC can?

Any help would be great as running out of ideas.