It sounds like you are wanting to determine the cross-sectional area at any section, instead of the volume or you are trying to determine the volume between any two sections. Create a TIN volume surface and then check out the XML reports for Volume Surfaces. I don't recall if there is one that will list the area at each section or volume between two sections, but there may be. I cannot look at the moment.
Danny Counts
L.A. CAD, Inc.
http://digginginc3d.blogspot.com/