Is ní acceptable in place of níor?
Not here. Ní is the present/future/conditional tense negative marker, níor is the past tense. However, there are 6 irregular verbs that do use ní in the past tense.
Is d'eist tu how you would say you listened?
(Re-reading this answer some time later, I see an ambiguity in the way the question is phrased.
D'éist tú leis an gceol - "you listened to the music"
"You listened?", meaning "did you listen?" would be ar éist tú?
Is "he didn't hear" an acceptable translation? Rather than 'listen'? Anyone know?
"he didn't hear" is níor chuala sé.
Clois is the verb for "hear", éist is "listen".
Why not d'éist sé? Does níor mean the d' isn't necessary?
Yes. The do that used to indicate the past tense only survives as d' before vowel sounds (some speakers of Munster Irish still use it for all verbs), and is not used when you have a negative or interrogative particle.
The r in níor and ar indicates the past tense.