_Récupérez the inner outline (right-click on one of these entities, "Select Loop", if necessary click on the yellow arrow to change the orientation then Tools/Sketching Tools/Convert Entities)
_Extrudez this sketch by unchecking "Merge Result" and selecting "All the way to body", clicking on your original body.
This gives you a body that represents the volume of the interior of your room.
Select it in the tree (density folder) and then, Tools/Mass Properties, you will see its volume.