Question: What is considered loud recitation or quiet recitation? Should I take the opinion that merely moving the lips is sufficient as recitation, if I fear reading too loud in the silent prayer?
Answer:
In the Name of Allah, the Most Merciful and Compassionate,
Quiet recitation is loud enough that one can hear themselves. However, it is still considered within the scope of quiet recitation as long as the first row of followers behind one could not all hear the imam. [Ibn ‘Abidin, Radd al-Muhtar citing al-Khulasa]
Even if one is not leading a congregation, slightly raising the voice does not constitute loud recitation.
Various Opinions
There are two valid opinions with regards to quiet recitation. The first and superior is that one recites loud enough that one can hear their own voice. The second position is that merely moving the lips in pronunciation of the letters is sufficient. [Ayni, Binaya Sharh al-Hidaya]
Due to the presence of these two valid opinions, it is recommended for you to act upon the superior position (i.e. reciting loud enough that you could hear yourself). [Ibid.]
Summary
The prayers that you previously performed by merely mouthing the letters without sound are valid and no make required. It is recommended to act on the superior opinion from here on, however not necessary.
I hope this helps,
Allah knows best.
[Shaykh] Yusuf Weltch
Checked and Approved by Shaykh Faraz Rabbani