Answer:
Sound Waves only needs a medium which in this case is Air,not withstanding the wall demarcation.
Explanation:
Sounds Waves can't be seen,they are Invisible.Anytime a person talks the produce vibration,and anytime you see vibration you are definitely seeing sounds.
You hearing your friend talk to you from the other compartment is made possible because their is a medium which in this case is Air.
Sound waves can travel in other medium apart from Air,Such Medium Includes Water and solids.
Air is invisible and when you talk you are sending sound waves into the air which in turn causes vibration which in this case you don't see but you can hear provided its of a reasonable pitch.