duo accepts both 'zij hebben niet van jou gehoord' and 'zij hebben niet gehoord van jou.' are these interchangeable, or do they have different connotations?
They're interchangeable, the former is a bit more common word order I think.
Probably it was a listening exercise.
They are only different in terms of emphasis and pronunciation.
• zij: emphatic pronoun, the ij sounds close to 'ay' in the English word May.
• ze: unstressed, 'regular' form. The e is pronounced with a schwa, which is the sound of the e in the English word differ.
So, if it's a listening exercise, you must type exactly what the voice says.