Question

As Salamu Alaykum:
Please keep up the good work.

My question is:

In Turkey, I heard the letter "Daad" pronounced very much like "Zaa" lots of times. Even people are taught like that.

My friends from Pakistan and Afghanistan reported the same tendency.

Is it acceptable? Is it acceptable in other Qiraah?
Do you have any idea why this is so common?

Jazakum ALLAHu Khairan

Answer

Wa alaikum assalaam wa rahmatu Allahi wa barakatuh.  May Allah keep our intentions solely for Him and guide us all in all that we do.

Your observations are correct, some Muslims in different parts of the world mispronounce the letter  and it sounds more like a "Zaa" than the correct sound.  This is not acceptable, and those of knowledge need to explain to those that do not have the knowledge the correct way of pronouncing this letter (and any other mispronounced letters).  This type of mistake falls into the category of  , or clear and obvious mistakes.  These are mistakes that are not acceptable and change the Qur'an.  Indeed, these mistakes are serious and ones that must be avoided.

There is no qira'ah that pronounces the letter  like a Zaa'.  All of the qira'aat pronounce the letter  the same way. 

It is a common mistake because  is a letter unique to the Arabic language and one that needs a bit of practice for the non-Arab to perfect, but also the colloquial Arabic, and most likely the other languages use the sound you referred  to  ( a Zaa with tafkheem) in their everyday speaking in place of the  . 
The Qur'an teachers have to be diligent in their corrections and make sure the students know their mistakes and the seriousness of them.  This is a great responsibility that they have; may Allah bless them and make easy for them their task.

Wa iyyakum wa-l-muslimeen.