What is the ruling on the prayer of one who finds difficulty in pronouncing letters when reciting Al-Fatihah and Al-Tashahhud, and is it permissible for him to raise his voice during prayer to ensure correct pronunciation?

It is disliked for a worshiper (ma'mum) to raise his voice when reciting during both audible and silent prayers. This is based on the Prophet, peace and blessings be upon him, who said: "Let not some of you raise your voices over others with the Qur’an." This dislikeness is emphasized if it disturbs other worshipers. Therefore, the worshiper is advised not to raise his voice during recitation, and to recite the Qur'an naturally without over-scrutiny. If he doubts a mistake in recitation, he should not pay attention to that doubt as long as he is not certain that it changes the meaning.