Is it obligatory to raise one's voice in an audible prayer when praying alone, and what is the ruling on the Adhan and Iqamah in that case?

The Prophet, peace and blessings be upon him, used to recite aloud in the two rak'ahs of Fajr prayer, and in the first two rak'ahs of Maghrib and 'Isha' prayers. This audible recitation is a Sunnah (definition). If a worshipper recites silently in a place where audible recitation is prescribed, their prayer is valid, but they would have abandoned a Sunnah. It is permissible for a person praying alone not to say the adhan (definition), unless they are in a desert or a remote farm, in which case it is legislated for them to say the adhan and iqamah (definition), even if they are praying alone.