ECHO

It sounds like there may be multiple open mics and/or speakers in the room.  Not only do you need to make sure the mics are muted, but you also need to make sure all speakers (with the exception of the hosting computer) are muted as well.  You do not want to have more than one mic/speaker active at any one given time.  Check out the link below.  

https://support.zoom.com/hc/en/article?id=zm_kb&sysparm_article=KB0061720#:~:text=Generally%2C%20if%20you%20are%20hearing,unmute%20one%20at%20a%20time.