What isosurface and isovalue are doing are generating a surface around those points that are above a certain value. All code you showed is good for visualising, but what you really want here is not a visual display but a number of the volume.
If you want the volume that is the same as how many points satisfy your condition of voxel intensity > value, so what you have to do is the following:
volumeOfInterest = (V >-2 );
That will generate a matrix of same dimensions as V with 1 for those points above the isovalue and 0 below, then simply sum those values
volume = sum (volumeOfInterest(:));
And then, if needed you will need to callibrate that value to the physical quantity of your interest (say mm^3) with a conversion between voxels and that quantity.
Hope that helps