Is it considered showing off (riya') to try to lower one's voice in a silent prayer (ṣalāt sirriyyah) out of fear of being heard by others, and does the prayer become invalid thereby? Is it normal for others to hear the worshipper's voice in a silent prayer? And is it obligatory to continue lowering the voice?

What is prescribed for the worshipper is to hear himself only, especially if there is someone next to him who would be bothered by the loudness of his voice. This is what the Prophet, peace and blessings be upon him, did when he heard the Companions reciting loudly, so he forbade them from doing so. Therefore, lowering the voice is not hypocrisy; rather, it is in accordance with the Sunnah (definition). One should not be excessively preoccupied with this, so that it does not distract from contemplating the remembrances of the prayer. And one must cast away the whispers of showing off, because giving in to them leads to great evil.