As it is defined that we need to pray at low Loudness extent in Zuhur and asr salah and different ruling for different salah , but what are the rulings for nafl and Sunnah prayer of 5 times
All the optional prayers (sunnah or nafl prayers) are recommended to be performed in silence (especially the day-time prayers). This might be due to the fact that one is closer to Allah, so one shouldn't "show off" by for example reciting in a good and loud voice to attire the attention of others and should concentrate on the worship.
But it is allowed to pray them in normal our low voice too -if necessary- for example if praying in congregation as it is usually done in taraweeh, tahajjud and Eid prayers and sometimes in shaf'a and witr.
It says in al-Mawsoo’ah al-Fiqhiyyah (25/281):
What is mustahabb and makrooh in Sunnah and naafil prayers:
It is mustahabb to recite quietly if the naafil prayer is offered by day, following the ruling on the daytime prayers. He has the choice between reciting out loud or quietly in the night prayers, if he is praying alone, but reciting out loud is better, provided that it does not disturb others. But if he is leading others in the naafil or Witr prayer, then the imam should recite out loud so that those who are praying behind him can hear him. But the one who is praying alone should be moderate in reciting out loud. (source: the islamqa fatwa linked below)
(For further information see islamqa #91325)