Apologies for the previous long email. I had previously tried your suggestions - the results being the same on the OM demo site below:
I tested the following mute functionality on the https://om.alteametasoft.com/openmeetings demo site also and noticed the following (with 3 users):
SHIFT + F7 (works as expected as a quick on / off toggle for individual users)
SHIFT + F12 mutes all users (including moderator)... however, moderator and all participants will have to click individually on the "speaker /audio" of all participants if they want to hear each other again.
Perhaps it would be more useful to mute everybody... except the moderator... whereby everyone can at least hear the moderator.
Finally (the function I am really eager to use), clicking on a users video pod (or avatar when they have their camera deactivated) in order "mute microphones of others"... will result in the popup message "Do you really want to mute all users except this one?" ... but does not do anything. As I can't test this any further, I can only assume that would this mute the moderator also and if so would it best to "mute all others except this one and the moderator".
I tested this using the latest versions of Chrome and Firefox. When using Firefox the current hotkeys open the Firefox web debugging tools also.The room type I was using was "conference" (https://om.alteametasoft.com/openmeetings/#room/98659).