It is recommended to be in a moderate voice, neither too loud nor too silent, somewhere between sirri salah qirat and jehri salah qirat.

It is evident from various narratives that initially in this prayer, the Quran was at times recited loudly and at times silently. Later, the Almighty directed the Prophet (sws) to recite it in a medium pitch:

وَلاَ تَجْهَرْ بِصَلاَتِكَ وَلاَ تُخَافِتْ بِهَا وَابْتَغِ بَيْنَ ذَلِكَ سَبِيلاً

And in this night prayer of yours, pray neither with too loud a voice nor in a low one, but, between these extremes, seek a middle course. (17:110)

Consequently, the Prophet (sws) also directed his Companions (rta) to recite in this way. Abu Qatadah narrates that the Prophet (sws) told Abu Bakr (rta): “When I passed by you, you were reciting the Quran in a low tone [in the night prayer].” He replied: “I recite it to Him Who hears [even my] whispers.” The Prophet continued: “Raise your tone a little.” Then he told ‘Umar (rta): “When I passed by you, you were reciting the Quran in a very loud tone.” He replied: “I awake those who sleep, and make Satan run away.” The Prophet said: “Lower your tone a little.” (Sources: https://sunnah.com/abudawud:1329 , https://sunnah.com/tirmidhi:447)