What is the ruling on the authorized person reciting the marriage contract sermon and supplicating for the groom and his wife?

It was not transmitted from the Salaf (pious predecessors) that they recited hadith and supplications in a melodic, chanting manner like the Qur’an, so it is better to avoid that.

If the sermon is the Khutbat al-Hajah (Sermon of Need) narrated from the Prophet, peace and blessings be upon him, then it falls under prophetic hadiths. If it is other than that, then it is even more appropriate to avoid reciting it in a melodic, chanting manner like the Qur’an.

As for the recitation of the sermon in the sense of reading it with deliberation and slowness, there is no harm in that; rather, it is desirable, as long as it does not lead to undue prolongation.

The rules of Tajweed should not be applied to anything other than the words of Allah Almighty. However, there is no harm in reciting hadith with good voices without applying the rules of Tajweed, and it may even be more conducive to listening and understanding.