What is the maximum distance one may be from the mosque and pray individually while still hearing the voice of the imam and the khateeb, such that the worshipper still receives the reward of congregational prayer in the mosque?

The prayer of an individual outside the mosque, while hearing the imam's voice, is considered valid by some scholars and is not invalidated by the majority. The individual receives the reward of congregational prayer, but it is less than the reward for praying inside the mosque with the rows of worshippers. It is preferable to pray inside the mosque.