Please it allowed saying Salaat upon the Prophet aloud when the Imaam mentions his name in recitation during prayers (Salaat) or one should say it without being audible. And can one say the affirmation after Surah tin aloud as well?

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 is His slave and Messenger.

It is permissible to exalt the mention of the Prophet during the prayer when the Imaam recites a verse in which there is a mention of the Prophet . The same thing applies when the Imaam recites a verse in which there is exaltation (of Allaah), one may say 'Subhanallaah', or a verse where there is a mention of mercy, one could supplicate Allaah to have mercy upon him, or a verse where punishment is mentioned, one could supplicate Allaah to save him from punishment and from Hellfire. When one hears the verse of Allaah (which means): {Is not Allaah the Best of judges?}[Qur'an 95:8], he may say: "Yes indeed, and I testify to that."

For more benefit, you could refer to Fatwa 82963.

It should be noted that this is a general ruling that applies to the Imaam and the people led in prayer, and whether that is in obligatory prayer or in supererogatory prayer. However, the person led in prayer should not raise his voice and disturb other praying people.

Allaah Knows best.