I'm hearing an echo on the HoloLens, how do I fix that?

The echo is being itself is being caused by the expert's PC, not the HoloLens. This occurs in rare instances where there is a conflict between WebRTC and unique hardware configurations on the PC.

There is a solution.

On the expert's PC open Control Panel then Hardware and Sound

Navigate to Sound

Navigate to the Recording tab, find your microphone, and click Properties

Navigate to the Advanced tab and uncheck Enable audio enhancements

Click Apply or OK