For me, it is - they were talking in the garden. This is native for me and where I come from. If someone is speaking - it is one person. She was speaking to a neighbour in the garden - implying one person was speaking and one person was listening. e.g. Can I speak to you for a minute, please? Meaning - I want to say something to you and I want you to listen. Talking implies a conversation - both people speaking and both people listening. e.g. Can I talk with you a minute, please? Meaning - I want to have a conversation with you about something. However, you would not be misunderstood if you used 'speak' instead of 'talk' - they are frequently misused in conversation even by native speakers.

There isn't a real difference. Talking and speaking are synonyms.