In D&D 5th edition, two principles which apply when adjudicating rules:
- The rules mean exactly what they say and no more.
- Anything not covered in the rules is decided by the DM.
Since there is no rule specifying otherwise, then, officially, adjudicating whether sound is "physical" for the purposes of a wall of force is entirely up to the DM.
However, if you want to be able to talk to someone you trap within a hemispherical wall of force, simply create the wall one inch off the ground. The spell allows the wall to be free-floating, and the gap would allow speech.
Additionally, unless the dungeon floor is also a wall of force, it's not perfect; it's going to have gaps and cracks that would allow sound. It may also be possible to hear someone through the connecting stone floor if you shout loud enough (imagine listening to someone through a wall).