For blocks only, hexahedral cell volume is computed using the method of Davies and Salmond (Ref. 6) which uses pyramid decomposition. Tetrahedral cell volume is computed by calculating the cross product of two of the cell's edges, taking the dot product of that amount, and multiplying the result by 1/6.
Tip: Looking at your grid's cell area or volume is a quick way to see if you have any cells that are approaching zero in size. Often these cells arise because of areas in the topology of your mesh that constrain the cells in that region to extremely small sizes. Small tweaks to the topology in these areas will usually fix these problems.