Is it appropriate to mute others in a video call

I think the most appropriate thing to do is to mention in the call that a person is unmuted. Depending on culture you can mention who is unmuted. You can do this no matter if you are the organizer or not, if it's a problem for you it's most likely a problem for other persons as well.

That depends why they were unmuted. Are they actively participating at the time, or did they just forget to go on mute when they were done speaking?

As long as the participant you muted wasn't actively speaking at the time (i.e. they just forgot to mute), I don't think that there's any problem with it. In fact, if I forgot to mute, I'd personally prefer it if someone else muted me; if I had something going on in the background, I don't necessarily want all of my colleagues to hear it.

On the other hand, if they were actively participating at the time, it would most likely be rude because it could imply that you didn't want to hear what they had to say.

Note that at least Zoom lets you (as the organizer) select that everyone starts muted by default when they join the meeting.

That is in my experience the least controversial approach, because it teaches people that being muted is the norm, not a punishment for background noise. There's no need to interrupt the meeting to call people out and ask them to mute themselves or to risk offending someone by force-muting them.

People tend to learn very quickly that they need to unmute themselves whenever they want to speak. Muting yourself once you are done speaking then also comes naturally. Essentially, everyone just adapts to a "push-to-talk" mode.

Yes.

Speaking as a participant, I think it is useful in your scenario. I once adjusted my webcam in an open discussion forgetting to mute myself and my boss muted me. I found it totally okay and was sorry for not muting myself. I could unmute myself afterward.

It is another case when you mute someone, because he should not speak. Then there is a conflict about who was about to speak that usually should be resolved by the persons "oh, you were talking" and may need some clarification on team call etiquette when the problem happens more often.

When muting because of technical issues like mic feedback loops, you should probably send a short chat message describing what you heard, so the person can try to solve the technical problem on their side (e.g. switching from speakers to headphone).

As the meeting organizer, it would certainly be within your scope to mute the offending party, and you should send them a private message notifying them of that so that they don't attempt to speak and are thus excluded from the meeting. It's not great etiquette, but the meeting must continue without disruption.

The technique we use in our company is to pause the meeting, announce everyone needs to mute, and then if they continue unmuted they are called out by name and instructed to mute. We have yet to go beyond that level of attention on it. Most people are generally willing to mute, and the ones that don't are usually under the impression they are muted (though aren't).

My personal preference is the second one for two reasons. The first is that it signals the intention that the meeting is a serious event and should be paid appropriate attention. The second is that it establishes an order and respect to the group at large that will lead to a better meeting experience. When people see a leader taking the event seriously, they will be more inclined to participate in a serious and engaged manner.

Absolutely appropriate if you are the call organizer to mute someone in this situation. I think it would also be polite to inform them on the call or via chat so that they are aware that they have been muted and perhaps why.

In my opinion, the number of people on the call is irrelevant; if noise is disturbing the call then there is nothing wrong with muting them.