This is more complicated. I knew about "Tu stai parlando." The question is whether "Tu parli" can also be translated as "You are speaking." Here's a place that translates the Italian present tense with the English progressive present: http://quizlet.com/19220958/presente-dei-verbi-in-are-present-tense-of-are-verbs-flash-cards/ But I rely mostly on this: "The difference between, say, Legge il giornale and Sta leggendo il giornale is not necessarily that between English "She reads the newspaper" and "She's reading the newspaper." In modern colloquial English, if the event described is simultaneous with the time of speaking, there is no simple alternative to saying "She's reading the newspaper"; but Italian allows either Legge il giornale or Sta leggendo il giornale, using the latter only to underline the progressive, developing nature of the activity. Similarly, in the past, "She was reading" corresponds both to Leggeva il giornale and to Stava Leggendo il giornale." from (Maiden and Robustelli, A Reference Grammar of Modern Italian, p. 303). Of course, I'm not a native speaker, and I defer to those who are.
I think "You are talking" should be accepted.
For the most part the English present continuous (ie. you are talking or I am eating, etc) is translated to Italian in the simple present (ie "Tu parli" or "io mangio"). I can't see any reason why it wouldn't be accepted here, so I expect that it's just an oversight on Duolingo's part.
Yes, you are correct. If you're a native English speaker, an Italian p sounds like a English b to your ear. That's because Italian p doesn't have the extra "puff" that accompanies the English p.
Instead Italian relies on the difference between voiced and unvoiced only (that it: the vocal cord vibrating or not).
The same applies to t (unvoiced) and d (voiced).
Your ear is simply not trained to Italian sounds :-)