In order for Webinar Attendees to unmute, they would first need to be allowed to talk. Please note this can only be done for a single attendee at a time, so it would take some time to allow this for a larger event.
Generally speaking, Webinar Attendees are only allowed to view the content of the event, while Panelists have the ability to talk. This prevents disruptions and background noise. I suspect this is also why the feature worked in your test session, as Panelists do not need to be allowed to talk.
If attendee audio participation is important, I would suggest using Zoom Meetings instead of Webinars. Otherwise, you might consider using Webinar Reactions which has a Clapping hands option and would provide visual feedback to the current speaker.
You can also check this article for more information about the various Webinar options: https://support.zoom.us/hc/en-us/articles/115004834466-Managing-attendees-and-panelists-in-a-webinar
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