What is the effect of changing the pronunciation of some letters—such as the Qaf and Ta from voiced to unvoiced, and the Dad and Jim, whose characteristics linguists have differed on—on the recitation of the Holy Quran today, and is it permissible to excessively whisper a letter that we know is unvoiced?

The inherent characteristics of letters do not change, for each letter has a makhraj (point of articulation) and attributes that distinguish it from others. Changing a letter's makhraj transforms it into another letter and is considered a manifest, forbidden error (laḥn jalī). An example is extending the Ḍād until it becomes a Ẓāʾ, which changes the meaning. Introducing an attribute that does not belong to a letter, such as qalqalah (vibration) for the Nūn, is considered a subtle error (laḥn khafī), unless it alters the letter itself. Exaggerating an attribute, such as hams (whispering) to the extent that it resembles another letter, is a manifest error. The principle in recitation is not to be overly meticulous; therefore, changing a letter into another letter is forbidden, and if it does not change the letter, it is disliked (makrūh).