Answered by Sidi Sulaiman Hart
For six years I have been praying without whispering the surahs, instead I use to just recite the surahs in my head. Do I have to make-up those prayers? In addition, for the make-up prayers, can I pray them anytime? When is the best time to pray the make-up prayers?
In the Name of Allah, Most Merciful and Compassionate
It is a condition for the validity of the fatiha ( and all other verbal integrals) that one would be able to hear its recital were one in a quiet environment, so if you failed to do so for the fatihah, you failed to validly fulfill that integral and thus failed to validly fulfill the prayer.
This is the ruling in the Shafi`i school, so if one wishes to follow it, one must repeat all those prayers in which one failed to validly recite. You may wish to investigate the validity of your prayers in the other schools. You can ask Shaykh Faraz Rabbani at the Hanafi Group.
If you did not hear yourself recite a sunna then the prayer is still valid and does not need to be repeated.
You may make up prayers at anytime of the day, including the times in which it is usually haram to pray.
(See : Bushra l-karim, page 66 (Mustafa Babi al-Halabi))