You can simply calculate how many cells (voxels?) in your labels-variable are 1:
Voxels = sum(labels(:)==1)
That is the number of elements in labels that are 1, if each cell corresponds to voxels of the same size you just need to scale that sum with the voxel-volume. If the voxel-volume varies you'd have to take that into account too:
VoxelVol = sum((labels==1).*dV,'all');
where you'd have to supply a 3-D array dV of the same size as labels with the volume of the individual voxels.
HTH
