Can I raise my voice in order to help me concentrate in prayer? What if it is a nafl prayer, or if I am praying alone?
In the name of Allah, the Inspirer of truth
It would be permissible to raise the sound of ones recitation to retain the concentration.
This is also permissible in prayers that are normally performed in a subdued tone. Besides the need in the case in question, it is totally permissible for a person praying alone to recite aloud in any prayer he is performing, and it is not an obligation to recite low even in the inaudible prayers like Zuhr and ‘Asr. This is only an obligation upon the imam according to the most correct opinion. (see Radd al-Muhtar 1:498)
Mufti Abdurrahman ibn Yusuf