This video demonstrates the ability to change the units displayed from the Analyze Volumes command.  The units display is coming from the Working Units of the Model which is active and containing the terrains.  You can change the working units of the model and the output of the report will reflect that change when you run the Analyze Volumes command.

NOTE:  If you create a secondary model for a different unit output, keep the original working units when importing the data, then go change the working units of the Model.  This is necessary to import the data to the correct size and location if coming from a unitless file (i.e. InRoads DTM).