Dought & clear, - Sifat al-Salaah (description of the prayer) ->Reciting the opening takbeer quietly in a prayer in which Qur’aan is to be recited out loud.

If I say the opening takbeer quietly in a prayer in which Qur'aan is
to be recited out loud, is my prayer valid?.
Praise be to Allaah.
It is Sunnah to say the takbeer out loud and to recite Qur'aan out
loud in a prayer in which Qur'aan is to be recited out loud, but it is
not obligatory, so if a person says them quietly there is nothing
wrong with that and his prayer is valid.
But you have to move the tongue and form the words, and they cannot be
uttered without doing that, whether in a prayer in which Qur'aan is to
be recited out loud or one in which it is to be recited quietly.
Some fuqaha' stipulated that the worshipper should be able to hear his
own voice, but the correct view is that it is sufficient to move the
tongue and form the words.
The details of that opinion are as follows:
1 – The majority of Shaafa'i and Hanbali scholars, and Hanafi scholars
according to the more correct of their two views, are of the opinion
that it is obligatory for the worshipper to utter the takbeer in such
a way that he can hear his own voice, and it is not sufficient for him
to move his tongue without making a sound. The same applies to all
dhikr; it does not count if there is no sound.
2 – Some of the scholars are of the view that it is acceptable to move
the tongue and form the words without making a sound. This is the view
of the Maalikis and of the Hanafis according to the other report. It
was also the view favoured by Shaykh al-Islam Ibn Taymiyah. Ibn
Qudaamah (may Allaah have mercy on him) said inal-Mughni(1/276): The
worshipper must be able to hear himself [i.e., saying the takbeer]
whether he is leading the prayer or otherwise, unless he has a
problems, such as being deaf, or something is preventing him from
hearing, in which case he should do it as if he could hear or as if
there was no problem. Because dhikr is something that is to be done
verbally, and there can be no speech without sound, and sound is what
can be heard, and the closest of listeners to him is himself, so if he
cannot hear it then he does not know whether he has said the word.
There is no difference between men and women with regard to what we
have said. End quote.
Al-Nawawi (may Allaah have mercy on him) said inal-Majmoo'(3/256):
The minimum form of reciting quietly is such that a person can hear
himself if he is sound of hearing and there is no problem such as
surrounding noise and the like. This is general in meaning and applies
to recitation, takbeer, tasbeeh when bowing and so on, tashahhud,
salaam and du'aa', whether the prayer is obligatory or naafil; nothing
of it counts unless he can hear himself, if he is sound of hearing and
there is no problem. End quote.
See:Tabyeen al-Haqaa'iq(1/127) andal-Bahr al-Raa'iq(1/356).
Khaleel (may Allaah have mercy on him) said in hisMukhtasar: … and
reciting al-Faatihah by moving the tongue is a must for the one who is
leading the prayer and the one who is praying by himself, even if he
cannot hear himself. End quote.
Al-Hattaab (may Allaah have mercy on him) said: Ibn Naaji said inSharh
al-Risaalah: It should be noted that the minimum form of reciting
quietly is that one should move the tongue whilst reciting, and the
maximum is when he can hear himself only. The minimum form of reciting
out loud is when he can hear himself and those who are close to him
can hear him, and the maximum is limitless. End quote. InSharh
al-Mudawwanahit adds: Whoever recites in his heart whilst praying is
like one who does not recite at all. Hence it is permissible for one
who is junub to recite in his heart. Ibn 'Arafah said: Sahnoon ibn
al-Qaasim heard that moving the tongue is acceptable for one who is
reciting quietly, but it is better if he can hear himself. End quote
fromMawaahib al-Jaleel(1/535).
Al-Mardaawi (may Allaah have mercy on him) said inal-Insaaf(2/44): The
words "reciting as much as he can hear himself" mean that the
worshipper must recite aloud in prayers where Qur'aan is recited
quietly and say the takbeer and so on in such a way that he can hear
himself. This is our view, and it is the view of our companions, and
it was stated definitely by most of them. Shaykh Taqiy al-Deen [Ibn
Taymiyah] favoured the view that it is sufficient to form the letters,
even if he cannot hear himself, and he mentioned it as a view in his
madhhab. I say: I favour this view. End quote.
Shaykh Ibn 'Uthaymeen (may Allaah have mercy on him) thought the view
of the Maalikis and Shaykh al-Islam was more correct. He said:
With regard to the words "he says", if we say that saying may be done
with the tongue, is it essential that he can hear himself saying it?
There is a difference of opinion among the scholars concerning this.
Some of them say that there must be a sound that he can hear himself.
This is the correct view; even if those who are next to him cannot
hear him, he has to be able to hear himself. If he speaks without
hearing himself, then there is no point in forming these words. But
this view is weak. The correct view is that it is not essential that
he be able to hear himself, because hearing is something extra to
saying and speaking, and if something is extra to what is mentioned in
the Sunnah, then the one who says that has to produce evidence for it.
Based on that, if a person is certain that he has pronounced the words
properly, but he did not hear himself, either because his hearing is
weak or because of noise going on around him, or for some other
reason, then the correct view is that all his words are acceptable and
he does not have to do more than what is indicated by the texts, which
is speaking. End quote fromal-Sharh al-Mumti'(3/25).
Based on that, if you moved your tongue and lips in saying the
takbeer, that is sufficient, but it is better if you say it in a voice
that you can hear.
And Allaah knows best.