The fatwa position in our times is that such mistakes in recitation do not nullify the prayer.
Ibn `Abidin states in his Radd al-Muhtar, the central reference for fatwa in our times:
“As for the later scholars… then they agreed that mistakes in declension (i`rab) never invalidates [the prayer] even if the belief [entailed by the change in meaning] is disbelief (kufr). This is because most people do not differentiate between the various declensional perspectives.”
A mistake in declension is to alter the final vowel of a word. Thus, if a word is supposed to end in a fatha but is pronounced as ending in a kasra, this would be a mistake in declension. As is clear from the words of Ibn `Abidin above, it would not invalidate the payer.
He continues by stating:
“If the mistake is in altering one letter for another, then if it is possible to differentiate the two [when reciting] without difficulty, such as the letter al-saad and al-taa’ when one recites al-talihat in place of al-salihat, then they agree that it invalidates the prayer. If it is not possible to differentiate, such as al-daad and al-zaa or al-saad and al-sin, then most of them state that it does not invalidate the prayer due to general hardship (`umum al-balwa).”
Based on this, common mistakes in pronunciation of certain letters is also overlooked. It does not invalidate the prayer even if there occurs a change in meaning. An example of such a mistake in pronunciation is the oft-occurring recitation of “walad ḍāllīn” as “walad zallin”.
To summarize, we can say that the following, when not done willfully, do not invalidate the prayer.:
a. Grammatical mistakes, such as those relating to the declension of words, and
b. Common mistakes in the pronunciation of certain letters and words.
Of course, we cannot stress enough the importance of learning correct pronunciation of the Qur’an. It is Allah’s book and among the rights due to it is to recite it as it was revealed, in a beautiful and sound manner.