Is a prayer valid if the phrase “ولا الضالين” is recited with a thick sound that changes the sound similar to the letter ظ in surah al-Fatihah?
I’m inclined towards that the prayer of a person is invalid when he purposely changes the recitation of a letter and it is also impermissible to pray with an imam who recites the Quran this way except when he can no longer change the way he recites it due to certain unavoidable reasons after he has tried his best to correct it.
According to Imam al-Nawawi, there are two opinions of scholars in this matter. The first opinion is that the prayer is invalid. This is the opinion held by Imam al-Haramain, Imam al-Ghazali stated in al-Basit and also Imam al-Rafi’i al-Qadhi Abu al-Tib, Syeikh Abu Hamid [al-Isfarayini] and others. The second opinion stated that it is valid due to the difficulty for the general public or others to pronounce the letter properly. 
The same opinion is shared by Syeikh ‘Atiyyah Saqar in the fatwa he issued.
“The correct way of the pronunciation of the letter da’ (ض) isn’t the same with the pronunciation of the letter za’ (ظ) although both of these letters are close in most of their characteristics except in terms of their makhraj and istitalah (lengthwise) characteristics. As for the ruling of changing a letter with another letter in al-Quran during prayer, the scholars have stated: If it is done purposely while he knows the difference between the two letters, then it is prohibited. Furthermore, some scholars even ruled it as kufur (denouncement of Islam) for he is changing al-Quran al-Karim. Thus, his prayer is invalid and so does his faith. Moreover, it is done not on purpose (ignorance), so it is obligatory for him to strive in understanding (learn) the correct way to pronounce the letter. If he didn’t do so (pronounce it correctly) when he is capable, then his prayer is invalid and so does following him when he is the imam. Whilst, if he is unable to correct his pronunciation then his prayer is valid and so does praying following him when he is the imam as stated by the majority of fuqaha’.” 
It is best to strive in correcting the pronunciation of each letter in the Quran by learning or taking a class with a competent teacher. Wallahua’lam.