The sound was not clear at all for "meu". I guessed at the "m" and the "eu" sounded more like the "el" in "mel" (honey).
Could minha be used instead in a sentence like this?
Yes....but only if you used "A gata"....."A gata e minha".
It is not my cat - whats wrong with this translation?
The translation is correct but since it says "O" meaning "The" you needed to write "The cat is not mine" I hope this helps :D
it's essentially the same, just changing the order of the words a bit and structure of the sentence... It is not my cat: Não é meu gato. The cat is not mine: O gato não é meu.