Sound issues zoom webinars

I am running webinars on a regular basis as the zoom owner and always with one panelist and normally around 150 attendees. I always have a test with the panelist (in webinar mode) before the webinar but more than sometimes the sound during the actual webinar is much worse than the test (which always are good). I have so far not required any headset or mic for the panelist. What is the right set up and tech requirements to avoid the sound problem (video is always ok).