Either 'talking' or 'speaking' is possible. I agree with everyone that 'talking' seems more likely, but you can't rule out 'speaking'. There is no reason why you couldn't say "They were speaking loudly".

I would say talking though honestly i think you could say either. Talking always involves multiple people in a conversation. Speaking can be on your own, or from one person to another in one direction (if you were lecturing someone or telling someone off). Speaking is more general. So talking is more informative and sliiiiiightly more natural.

I would say 'talk' for #4. There is only a subtle difference between talk and speak: talking is a usual conversation, while speaking is more when one person does most of the talking and another person/other people listen (where the word 'speech' comes from).

If someone is talking loudly, they are in a conversation and being loud. If somebody is speaking loudly that means they are using a loud voice.

It's talking. Speaking is one-sided.