I just say, in real life and here "what are you speaking of ?" and by chance it is accepted. I am not here to learn english nuances, let alone to translate to colloquial english or slang. For me, "May I have a beer" is not exactly the same as "Hey Mac, fill it up", even if the latter is heard a lot more often.
Language changes over time and nowadays I don't hear anyone say "about what are you talking?" The modern day sentence in English is indeed "what are you talking about?" Colloquially where I am from, you can also say "what on earth are talking about?", "what are you jabbering on about?" Or even "what are you banging on about?" But that's more if someone is talking nonsense or talking too much lol often times it's said playfully.