What you should when you enter the masjid and hear someone reciting the Holy Quran loudly?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

If you enter the masjid and find someone who reads the Quran loudly, then go near him, greet him with Salaam, and clarify to him in a soft and gentle manner that what he is doing is contrary to the sharee'ah, as he should not disturb the worshippers and harm them with his loud recitation.

Remind him that the Prophet, sallallaahu ‘alayhi wa sallam, came out to the people who were performing the prayer while reciting the Quran loudly and said to them, “The worshipper (the praying person) is conversing with his Lord, so let him concentrate on what he is saying to Him, and one of you should not recite Quran loudly in a way that disturbs the other.” [Ahmad; Al-Albaani: authentic]

It is most likely that if you follow this method in advising him, he will take heed, Allaah willing, otherwise you would have done what is required of you and will be acquitted from sin.

Allaah knows best.