Mukhtasar Muslim

Sayyiduna Ibn Abbas (may Allah be pleased with them both) says about this verse: "Do not recite your prayer too loudly, nor too quietly, but seek a way between," that it was revealed in Makkah Mukarramah at a time when the Messenger of Allah (peace and blessings be upon him) was hiding in a house due to fear. The incident is that when the Prophet (peace and blessings be upon him) led the companions in prayer, he would recite the Qur'an aloud. So when the polytheists heard the sound of the Noble Qur'an, they would abuse the Qur'an, the One who revealed it (i.e., Allah Almighty), and the one upon whom it was revealed (i.e., the Messenger of Allah, peace and blessings be upon him). Upon this, Allah Almighty said to His Noble Prophet (peace and blessings be upon him): "Do not recite (the Qur'an) in prayer so loudly" that the polytheists can hear it, "nor recite (the Qur'an) so quietly" that your companions cannot hear it, "but recite (the Qur'an) in a moderate voice," between low and loud.

Umm al-Mu’minin Aisha Siddiqa (may Allah be pleased with her) says regarding the statement of Allah Almighty: (And do not recite your prayer too loudly...) that this was revealed about supplication (i.e., do not supplicate in a very loud voice nor in a very low voice).