Pronunciation of "o"
When the duolingo audio is played at normal pace the single letter "o" sounds like it is pronounced "oo", (like in "soon"). When it is played at the slow pace the audio clearly says "o" (like in "orange"). Does anybody agree?
When a word ends in "o", "os" with no accent it sounds like "oo" in English at any speed. Unless you are saying every single syllable in a word or in a dictation, then you pronounce it like a normal "o". Or if you are angry and you ask, for example, "Quando?". But we cant settle rules. Most of time we just pronounce like "oo" (at the end of a word). So, in 'quantos carros?' And 'quanto carro!' the letter "o" sounds the same. As a help, listen to "Quando o sol bater na janela do teu quarto - Legião Urbana". In this slow song you can notice how "o" is pronounced many times at a slow speed ;)
This happens in everyday conversation too. When you're on the telephone and the other person doesn't understand what you say, then you speak slowly and pronounce every syllable by itself, causing you to change each syllable from a "sílaba átona" (unstressed syllable) to a "sílaba tônica" (stressed syllable). In Portuguese, when the vowels E and O are unstressed, they are pronounced like I and U, and when stressed, they are pronounced like an open or closed E and O, depending on the accent you put over it. When they are actually unstressed and you make them stressed by dividing each syllable, normally you would pronounce them like the closed variants (Ê and Ô). I hope I didn't make it more confusing :)
Please see this lesson from the PodCast Tá Falaldo
Lesson 9: Pronunciation of Vowel Raising, Parking Meters http://www.coerll.utexas.edu/brazilpod/tafalado/lesson.php?p=09
This lesson looks at a tendency that linguists call "vowel raising" (sorry for the technical jargon). It refers to times when the pronunciation of [o] becomes [u] and the pronunciation of [e] becomes [i]. We've already seen it in the case of the 3 vowels sounds in word-final position (vive [i], viva [ə], vivo [u]). However Portuguese, unlike Spanish, also has vowel raising in other instances: 1. [e] becomes [i] in pretonic syllables. This is a fancy way of saying that "bebida" is pronounced [bibidə], "espero" is pronounced [isperu], "preciso" is pronounced [prisizu], etc. 2. [o] becomes [u] in pretonic syllables. The same process occurs with words that are spelled with "o": "motivo" is pronounced [mutivu], "moeda" is pronounced [muedə], "comida" is pronounced [kumidə].
That's very helpful to check your spelling. The other day I realized my German teachers do a similar thing when we have a dictation.
It happens in names, too. My sister's name is Denise, we can pronounce it in two different ways in Portuguese: /zee/ and /ze/. Zee is the most usual, whereas ze sounds more official to me. The first time I transcribed her name into Japanese syllabary I got really puzzled but I ended up choosing /zee/ as that's how most people would call her.