What should I do when I get distracted during prayer by external sounds, so I don't know whether I recited Al-Fatihah or the Tashahhud, or uttered the transitional Takbir?

If you doubt the recitation of a word in Al-Fatiha, then recite that word and what comes after it. If the doubt concerns the final tashahhud (definition), which is a pillar, then perform it, because omitting even one letter from it invalidates the prayer according to some scholars. As for doubt related to the takbirat of transition or the first tashahhud, then perform what you doubted, because the default is not to have performed it. And if you do not perform it, it is said that sujud (definition) al-sahw (prostration of forgetfulness) is required, while it is also said that no prostration is required of you.