Walnuts and black
It can be sometimes difficult to distinguish "noix" and "noire" with the voice
Oh yes. Unless there's context - and there often isn't - it's very hard to distinguish.
Certainly on the slow track. "noix" will have no sound after the vowel, "noire" will have the French "r".
However the poor quality of the Voice (see in Questions section endless criticizms about the pronunciation), it is quite rare that an adjective (noir/noire) cannot be distinguished from a noun (noix). Even a short sentence should be sufficient.