Is it permissible to interrupt the Imam who has begun the prayer if he is not qualified, while there is someone more deserving than him, and is the one who interrupts him considered sinful, and what is the expiation for that?

The one most entitled to lead the prayer (imamate) is the one most knowledgeable of the Book of Allah.

If the Imam has begun the prayer, it is not permissible for anyone to pull him back and put someone else forward, unless he does not recite Al-Fatihah well and makes a mistake that changes the meaning, and he is able to correct his mistake but does not do so. In such a case, it is permissible to defer him and put someone else forward, provided that discord (fitna) is avoided, or the congregant may pray individually.

As for someone who cannot correct his mistake, such as a lisping person, his prayer and the prayer of those behind him are valid, and it is not permissible to defer him.

The worshipers must, before commencing the prayer, put forward the one most entitled to lead them.

It is not permissible to defer an Imam merely because of a bad voice.