I'm trying out vMix call for the first time. I'm sending the Master audio out to the caller. However the volume at the caller is very messed up, garbled, overly compressed. On the vMix side, I can hear and see them just fine. Also, the remote caller can see video properly. They just can't hear anything.

I found a reference to advanced.vmixcall.com, but that did not fix anything. It did however allow me to easily reconnect, and for just a fraction of that first second, the audio is clean. Then it goes all garbled.

In this test I'm on the same network. Looking at the call stats, there are no errors.

What could be going on? How can I fix it?

I tested on 2 different client machines, both Mac Laptops.
Tested with Safari and Chrome. Same issue on both.
Headphones or no headphones, muted or unmuted .. still same garbled, underwater type distortion.
Again .. it sounds clean for 1/2 second when reconnecting using the reconnect button on the advanced endpoint.

Master level is acceptable, verified with headphones from the vMix machine itself.

Set the callers onto an extra Bus and send that to them. There might have been some phase deletions going on at your end.