Is the prayer valid for someone from whom the letter 'seen' (س) or 'saad' (ص) or some other letters are pronounced unclearly, while the origin of the letter is pronounced but there is a defect in its completeness, if he does not re-pronounce the letters that he pronounced unclearly, knowing that he is able to pronounce them without defect in completeness, but this happens out of forgetfulness?

If the question stems from whisperings (waswasah), then you should recite normally and ignore the whisperings. However, if you accidentally pronounce a letter incorrectly, displacing it from its proper articulation point without any whisperings involved, then you must re-recite the verse containing the error, ensuring that the error is indeed present to avoid falling into whisperings, as the default assumption is the correctness of the recitation.