The Grand Ayatollah Khamenei answered a question about not hearing the recited words in Salaat.
Question: Is there a problem if words are recited but they are not heard in Salaat?
The Grand Ayatollah Khamenei: Takbir, Hamd (Surah Al-Fatihah), Surah (after Al-Fatihah), and Zikr, and Dua must be recited in a way that the reciter hears himself/herself. And if due to hard of hearing or deafness, or a lot of noise cannot hear, must recite it in a way that if there is no obstacle, can be heard (by the reciter).
Source: khamenei.ir