Speak or Talk
"Speak" and talk" can mean "to use one's voice to say words or to have a conversation."
1. "Speak" is considered more formal than "talk."
Being formal might be due to:
A formal setting, like work.
If the speaker does not know you well and wants to be polite.
What the speaker has to say is important.
"I would like to speak to your manager."
"The guest lecturer will speak about the history of the postal service in Australia."
On the other hand, being informal might be due to:
A casual setting - some companies have casual work cultures.
If the speaker is a friend.
What the speaker has to say is informal even though they may not know you very well.
"Wally will talk us through how to use the new software."
"Are you free to talk?"
2. "Talk" implies a conversation or a discussion; "Speak" focuses on the speaker.
"The others started talking to each other when Eve started speaking about climate change."
When it comes to conversation or discussion, although "talk" and "speak" are interchangeable but plural nouns and pronouns cannot be used with "speak."
"I would like to talk to you."
"I would like to speak to you."
"Let's talk about it later."
Not - "Let's speak about it later."
When it comes to focusing on the speaker and relaying information, I think the difference between 'talk" and "speak" here is quite subtle in everyday English.
"Wally will speak on the importance of work-life balance."
"Wally will talk about the importance of work-life balance."
3. "Speak" is used to refer to languages.
"How many languages do you speak?"
Not - "How many languages do you talk?"
4. "Speak" is used on the phone.
In general, etiquette on the phone is usually more formal unless you are talking to a friend.
"May I know who's speaking?"
Not - "May I know who's talking?"