Not quite pronounced the same. It's a difference between tapped 'r's and rolling 'r's.
If the word starts with an 'r' or there is a double 'rr', then that 'r' should be rolled (relax your tongue against the roof of your mouth and blow; you should sound like a purring cat or motorboat). https://studyspanish.com/pronunciation/listen-and-repeat/letter_rr
Otherwise, if it's just a single 'r' in the middle or at the end of the word, then you should tap it (flick your tongue against the roof of your mouth like you're almost about to pronounce the letter 'l' instead). https://studyspanish.com/pronunciation/listen-and-repeat/letter_r
Same for me. Voice recognition software refuses to recogize"un" and "centuron" and the default options pop- up windows don't contain it. There full of spell and gramatical errors. This expends more of my time than if I had just typed in the answer, instesd of using this "time saving" technique.
The one 'R' in "caro" should be tapped (have your toungue almost flap against your pallete like your about to pronounce the letter L), https://studyspanish.com/pronunciation/listen-and-repeat/letter_r
while the double 'RR' in "carro" should be rolled (this sound's a little harder to make, but might remind you of a purring cat or motorboat engine) https://studyspanish.com/pronunciation/listen-and-repeat/letter_rr
That said, it still something you have to have a good ear for; I still have trouble differentiating the two sounds, but that's probably because I haven't been practicing in a while.
Duolingo doesn't have that as far as I know. You'll have to find another resource.
...I could probably recommend StudySpanish as a starting point:
The difference between them lies in their genders. "Un" is masculine and can only be attached to masculine nouns, whereas "una" is feminine and must come before feminine nouns, It's the same with most other adjectives, like "el/la", "bueno/buena", "rojo/roja". For example:
"un niño" = "a boy", "un hombre" = "a man"
"una niña" = "a girl", "una mujer" = "a woman"
Genders in Spanish won't always be this obvious though. "Un vestido" is a masculine noun for instance, even though it names a female piece of clothing, "a dress". It's best to look in a dictionary to confirm what nouns are which gender.
Spanish is very phonetic; every letter coresponds to only one or two sound, especially the vowels, which have one and only one vowel sound.
"Cinturón" is spelled like this: C - I - N - T - U - R - Ó - N (notice the accent above the o?). You can tell that its spelled this way because of how it's pronounced: 'seen - too - RONE'.
In English, whether the article you should use is "a" or "an" depends on the type of sound the following word starts with. If it starts with a consonant sound or a long 'u', you should use "a":
"a black cat" ('b' sound)
"a tall ladder" ('t' sound)
"a waffle" ('w' sound)
'a unicorn" (long 'u' sound)
If the word starts with a vowel sound that isn't a long 'u', it should be "an":
"an orange unicorn" (long 'o' sound)
"an itty-bitty spider" (short 'i' sound)
"an arrow" (long 'a' sound)
"an umbrella" (short 'u' sound)
Now Spanish does not work that way, like at all! Whether to use "un" or "una" doen't depend in any way on the starting sound of the next word. Instead it depends on the gender of the noun that the article is describing. if it's masculine you use "un"; if it's feminine you use "una".
"un gato negro"
"un unicornio (naranjo)"
"una araña pequeñita" you get the idea...