Praise be to Allah.
Reciting out loud in Fajr prayer and in the first two rak’ahs of Maghrib and ‘Isha’ is Sunnah for both one who is leading the prayers and one who is praying alone. If a person recites silently there is no sin on him, but that means that he is neglecting a Sunnah. If the person who is praying alone thinks that reciting silently helps him to focus more in prayer, then that is o.k., because it was narrated that when the Prophet (peace and blessings of Allaah be upon him) prayed at night, sometimes he would recite out loud and sometimes he would recite silently, as was reported by ‘Aa’ishah (may Allaah be pleased with her). For one who is leading the prayers, the Sunnah is always to recite out loud, following the example of the Prophet (S) and because that enables the congregation to benefit by hearing the words of Allaah, whether the prayer is fard (obligatory) or naafil (supererogatory).
And Allaah is the Source of Strength.