Webinar--Video Volume

We stream in-person lectures to at home attendees.  All technology including mics and the laptop go through a system that projects into the room and to the audience at home (through Zoom).  When our lecture speaker includes videos in the PowerPoint presentation, a few issues happen with the volume.  Attendees on Zoom are only able to hear choppy/underwater sound clips (it's like a mix of the speaker talking and the video--just jumbled noise).  Also, attendees are saying after the videos are played, the speakers volume gets really low for a minute then comes back to normal.  How can we 1. Have a video play with the volume from both the video and speaker audible, and 2. Regulate the volume levels so the speaker's mic doesn't dip after videos are played.  Thanks!