'card' as in a holiday card should be an accepted translation for the word carta
It shouldn't actually. A holiday card would be 'cartão'.
The word "minhas" is very unclear in he robot voice.
I thought it said ninas!
It is for the female voice but OK for the male voice
Sou and são, someone explain please
"Sou" is 1st person singular (I am = Eu sou); "São" is 3rd person plural (They are = Eles/Elas são; also Vocês são, even though it's the plural You).
"In the letter" = "na A carta"?
"Na" is a contraction for "em" + "a".
Is "As palavras na carta são as minhas" correct?
Yes, it is. Not mandatory in this case, except if you intend to differentiate your words from someone's else in the same letter...
I always here "... nas cartas sao minhas" instead of carta, and get marked wrong. Would it be a correct statement if there were many letters?
Yep, it would be...