When you run zoom the Sound Device Controller seems to change with the projector. So if you do not get sound from the other end of the presentation check this setting out.
Click on the up arrow next to the Microphone icon at the bottom left of the Zoom Screen
Where is says "Select a Speaker" try another in the list until you get sound. You could also go into the Audio settings.
Where you see I have highlighted the option box drop it down and change the device controller. You could also click on "Test Speaker" and when you hear a sound you should also get sound from the other side of the presentation.
I have also found that there is sometimes an issue with the microphone that is built in to the laptop. I am trying out an external mic (as in the one with a headset to see if this rectifies it. However talk back is a small use as long as the students can hear the lesson. If there is a casual staff supervising response can still be made through the chat box.