I don't think that there is a difference. You could use either of these discourse markers in the following situation: Person A: ...... and there'll be a lot more traffic, too.' Person B: 'Yes, you're right. And speaking about traffic, have you heard about the proposal for the new road? or ' Yes, you're right. And talking about traffic, have you heard about the proposal for the new road?

You wouldn't really say "talking of..." You generally "talk about" things, and "speak of" things, (saying "speaking about" would sound strange). Sometimes talk and speak can be used interchangeably, but it depends which word follows. So I could "speak to" OR "talk to" you, both would be correct.

Hi :) If your sentence starts with "Talking of A" or "Speaking of A", it means the same : concerning, in terms of, regarding of, on the issue of A... I don't know if it's something English people say a lot, though. If it's not at the beginning of your sentence, I think Karen may be right. I've found written articles with sentences using "I am talking of" and "I'm speaking of" but that's probably not used much when it comes to talking.