If he prays the naafil prayer of Fajr and Maghrib with his wife, can he recite out loud?
Question

Is it permissible that I recite out loud while praying the Sunnah of fajr or Maghrib leading my wife, or not?.
Praise be to Allah.The scholars (may Allah have mercy on them) said that what is prescribed is for the worshipper to recite quietly in naafil prayers during the day, and he has the choice between reciting out loud or quietly during the night, because both are proven from the Messenger (peace and blessings of Allah be upon him), but reciting out loud is better.  
It was narrated from Abu Qataadah (May Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) went out at night, and he saw Abu Bakr (may Allah be pleased with him) praying in a low voice, then he passed by ‘Umar ibn al-Khattaab (may Allah be pleased with him) who was praying in a loud voice. When they met with the Messenger of Allah (peace and blessings of Allah be upon him), the Prophet (peace and blessings of Allah be upon him) said: “I passed by you, O Abu Bakr, and you were praying in a low voice.” He said: I made the One with Whom I was conversing hear me, O Messenger of Allah. And he said to ‘Umar: “I passed by you and you were praying in a loud voice.” He said: O Messenger of Allah, I wake up the sleeper and drive away the shaytaan. The Prophet (peace and blessings of Allah be upon him) said: “O Abu Bakr, raise your voice a little.” And he said to ‘Umar: “Lower your voice a little.” Narrated by Abu Dawood and classed as saheeh by al-Nawawi in al-Majmoo’. 
It was narrated that Abu Hurayrah (may Allah be pleased with him) said: The recitation of the Messenger of Allah (peace and blessings of Allah be upon him) at night was sometimes low and sometimes out loud. Narrated by Abu Dawood and classed as hasan by al-Nawawi in al-Majmoo’. 
Al-Kasaani said in Badaa’i’ al-Sanaa’i (1/161): 
With regard to voluntary prayers, if they are during the day then they should be recited in a low voice, and if they are at night then he has the choice; if he wishes he may recite them quietly and if he wishes he may recite them out loud, but reciting out loud is better because naafil prayers follow obligatory prayers and the ruling on obligatory prayers is like that. End quote. 
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. End quote. 
It says in Kashshaaf al-Qinaa’ (1/441): It is makrooh for anyone who is praying to recite out during the day in naafil prayers, apart from the eclipse prayer (kusoof) and prayers for rain (istisqaa’). When praying naafil prayers at night, he should see which is better. If there are people in his presence or nearby who will be disturbed by his reciting out loud, then he should recite quietly, but if they will benefit from his recitation, then he may recite out loud. 
Al-Muhibb ibn Nasr-Allah al-Kataani said: It seems that “day” here means from sunrise, not from dawn, and night means from sunset to sunrise. End quote. 
The view of Imam al-Shaafa’i (may Allah have mercy on him) is that he should recite quietly in all the regular Sunnah prayers, whether by night or by day. 
Al-Nawawi (may Allah have mercy on him) said: With regard to Eid prayer, prayers for rain, Taraweeh and prayers during a lunar eclipse, the Sunnah is to recite out loud, with no scholarly difference of opinion. 
With regard to daytime naafil prayers, the Sunnah is to recite quietly, with no scholarly difference of opinion. 
With regard to naafil prayers at night, apart from Taraweeh, the author of al-Tatammah said: He should recite out loud. Al-Qaadi Husayn and the author of al-Tahdheeb said: He should recite in a moderate tone, somewhere in between reciting out loud and reciting quietly. 
With regard to the regular Sunnah prayers that are offered with the obligatory prayers, he should recite quietly in all of them, according to the consensus of our companions. End quote. 
It seems, and Allah knows best, that the one who offers a naafil prayer at night has the choice between reciting out loud and reciting silently. 
Based on that, there is nothing wrong with you offering the regular Sunnah prayers of Fajr, Maghrib and ‘Isha’ silently or out loud. 
It should be noted that offering the regular Sunnah prayers in congregation is not part of the Sunnah, but there is nothing wrong with offering them in congregation sometimes. This has been discussed in the answer to question no. 38606. 
And Allah knows best.