Is there a rule for when to use jej versus niej?
Yes, "niej" is used with prepositions, "jej" is used in other cases when you have to use Genitive.
Why not 'słuchaj' please?
It works.
is jej the only option or can I also use nie słuchaj ją? thanks.
It's the only option, because the "negated accusative = genitive" -rule always applies.
Is "co mówi" not the direct object of "nie wie"?
idk, I tried "ona nie wie czego mówi"