Create a new part in the assembly. A simple revolved thick cylinder (use synchronous primitive) that covers the area you want to measure and interferes on top of the other parts. Then go back to the assembly and use 'Subtract' (feature tab) to remove areas interfering with parts (use the sync option). You will be left with the area between your original parts. You can then edit that new part and use physical prop. (inspection tab) to get the info about the volume.