I am not aware of a direct method but you can create another Solid Block of same dimension, insert it in the part and do a cut operation. This shall give you the inner cube. Use Mass Properties dialog to get the volume of this block now...
Hope it helps!!!
Thanks rajat for your reply,
Thats the method we re following but we need to calculate it directly.
As in SolidWorks the volume that is calculated is the one of the filled material.
If the shape of the model is irregular then it hangs up and we need to create offset surface and knit all the closed features.
If you have irregular or complex internal faces why not try using Mold Design Tools.
Follow the method as described in the Help file under Mold Design - Insert Cavity.
Basically you create an assembly, insert including a dummy block bigger than the volume you require, then create a cavity in this block from the other part or parts in the assembly. You then have the volume you require, save the internal body only and extract volume info etc.
Hope this helps,